Is Snyder's claim about Soviet-German friendship propaganda posters (1939‒41) supportable?

There is no doubt that in 1940, the Soviet Union was supplying Nazi Germany with materials necessary for the latter's war effort, particularly petroleum, grain, rubber and manganese. Even if the posters are counterfeit, the fact remains that the Luftwaffe ran on Soviet oil in the Battle of Britain. See Economic agreement between Germany and the Soviet Union, signed at Moscow on February 11, 1940. For a summary of its terms, see the German Foreign Office memorandum of February 26, 1940, by Karl Schnurre, in Nazi-Soviet Relations, 1939–1941, p. 131.

I have not seen any Soviet posters featuring the pro-German propaganda during the one-and-a-half year non-aggression pact era (October 1939 to June 1941), but undoubtedly they existed. The pro-German propaganda is well attested by many scholars. For example:

The signing of the Molotov-Ribbentrop Pact reversed the tone spectacularly. The word fascist was eliminated and virtually overnight the press adopted a pro-Nazi point of view regarding Europe.... Hitler's speeches were extensively quoted in the Soviet press in 1939-1940, and the commentaries were favorable.

Nationalist Propaganda in the Soviet Russian Press, 1939-1941, by Ewa M. Thompson. Slavic Review. Vol. 50, No. 2 (Summer, 1991), pp. 385-399.

and began to publish Nazi speeches.

Evidently "publish Nazi speeches" is different from just quoting Hitler or any other piece of Nazi propaganda. Definitely Hitler's speeches were not available in their full form to the general public. Maybe they were published in small numbers for internal party reading, which is totally different thing.

People at public rallies occasionally misspoke, praising “Comrade Hitler”

It seems to be totally invented idea by Snyder. Or possibly it is based on just one mistake by someone, similarly to how one instance of methanol poitioning in the Red Army mentioned by Vasily Grossman in his memoirs made Anthony Beevor to make a claim that "Red Army soldiers who discovered methyl alcohol drank it and shared it with their comrades".

calling for “the triumph of international fascism.”

This is totally impossible, "fascism" was a heavily loaded slur word since at least 1936. Also, fascism never claimed to be an international movement.

Regarding the poster, it is evidently a fake because the USSR never was at war with Great Britain not to say, bombing London. Even at the hight of the Germany-Soviet relations the official position of the USSR was that they were neutral. Making such poster would be contrary to the stance of the USSR in foreign policy.

Short answer:

I have only read half of Snyder's books, but I think that OP's skepticism is misguided.

The OP is in error interpreting "posters" as propaganda items for mass-distribution, and the efforts for identifying them are on a wrong path. Snyder is not suggesting that there were cases where, for propaganda purposes, Stalin's figure was superposed with a swastika etc. The "posters" in question are in fact banners like those at the joint military parade of Brest-Litovsk where the leaders' effigies and/or specific symbols of the two powers were jointly displayed.

Long answer:

The fragment quoted is from an article to which I do not have full access, but it is similar to a passage in a book by Snyder that I do have, namely Black Earth - The Holocaust as History and Warning, chapter 7, Germans, Poles, Soviets, Jews (I have it as an e-book with no standard pagination):

In 1939 and 1940, the Soviet alliance with Nazi Germany sowed ideological confusion among Soviet citizens. The Soviet press ceased to criticize German policies and began to publish Nazi speeches. Soviet citizens in public meetings occasionally misspoke, praising “Comrade Hitler” when they meant “Comrade Stalin” or calling for “the triumph of international fascism.” Swastikas began to appear as graffiti in Soviet cities.

The fact that there is no direct scholarly reference for all those details is not bothering me too much, I can trust the scholar on this, namely that on the occasion of the Nazi-Soviet alliance:

• parts of Nazi speeches were published/quoted in the Soviet press
• Swastikas appeared as grafitti in the USSR
• Swastikas and communist symbols appeared together along the posters of the two leaders on certain occasions

In chapter 4, The State destroyers, we find this:

German and Soviet forces met at Brest and organized a joint victory parade, swastika followed by hammer and sickle, “Deutschland über Alles” followed by the Internationale. The Soviet commander invited German reporters to visit him in Moscow after the common “victory over capitalist Albion.”

The above is what is meant by the passage in the article that intrigued the OP: Swastikas began to appear on buildings or even on posters of Soviet leaders:

• On the occasion of the joint victory parade in Brest, there were joint posters of the Nazi and Soviet leaders and symbols (swastika and hammer&sickle); that doesn't refer to popular propaganda posters (meant for large distribution) like the ones the OP was looking for, but only to local and specific parade posters, probably meant to appear only once, then and there. But that doesn't mean that propaganda items of larger distribution celebrating the Nazi-Soviet cooperation were totally absent: see below on swastikas-material distributed (exclusively or not) to ethnic Germans.

That type of "posters" celebrating the rapprochement of the two powers must be imagined as obligatory in all joint festivity of that period. In the best known book by Snyder, Bloodlands. Europe between Hitler and Stalin, Chapter 3, National terror, p.116, we find this:

Ribbentrop made for Moscow, where, as both Orwell and Koestler noted, swastikas adorned the airport of the capital of the homeland of socialism. This, the final ideological shock that separated Koestler from communism, was really a sign that the Soviet Union was no longer an ideological state.

The larger passage makes reference in NOTE 58 to Haslam, Collective Security, 227 [meaning: Jonathan Haslam, The Soviet Union and The Struggle for Collective Security in Europe, 1933-39], Quotation: Weinberg, World at Arms, 25, [A World at Arms: A Global History of World War II] but mainly this refers to eyewitness reports made by both George Orwell and Arthur Kostler.

In NOTE 43, p.487 of the same chapter we find this:

The Soviet rationale was a classic one. First, the NKVD “established” that Germany had hundreds of spies among the Volga Germans. Then, the NKVD argued that the entire population was guilty, since none of the Volga Germans had reported all of this espionage to the proper authorities. In a particularly refined move, the NKVD used the presence of swastikas in German households as evidence of Nazi collaboration. In fact, the Soviets had themselves distributed those swastikas, in 1939, when Moscow and Berlin were allies, and a friendly visit from Hitler was expected.

That means that swastikas were used at some point during the 1939-41 pact in Soviet propaganda aimed at the Soviet ethnic Germans. When these people became victims of later persecution, those pro-Nazi propaganda items were unsurprisingly used against them.

I see nothing surprising here, although surprise is the main base of OP's skepticism.

That surprise is an impression produced by an article that is judged out of the context of Snyder's main argument in more than one book: that the Soviet-Nazi common (simultaneous) actions, but also subsequent (non-simultaneous) actions on the same territories (partition of Poland and other parts of Eastern Europe, but also the occupation first by the Nazi and then by the Soviet - or the other way around - of much of Eastern Europe) have to be considered the main explanatory factors for the specific (chronological and geographical) development of the most tragic events of the WW2 (the quasi-total destruction of the Jews, and the death of millions of Poles, Ukrainians, Belorussians, and Russians), as well as for the unprecedented realities of post-war Eastern Europe, which some now take for granted (the sudden general absence of the Jewish communities which had flourished there for hundreds of years, and the delimitation of new frontiers in relation to the ethnic cleansing of Poles of Ukraine, of Ukrainians of Poland, of Germans of Poland and Czekoslovakia, etc).