Upvote:5
The difference appears to be in the respective "messages" conveyed.
"Carrying a bag on the side" was seen as carrying a bag "temporarily." That was consistent with "white collar" work such as "geology," (which then required and still requires a college degree). Another example is that of a physician.
"Carrying a bag on your back" in the early 1900s signified "carrying ALL your possessions. (This was no longer true beginning in the 1960s.) A person who carried "all his possessions" on his back was seen to be a poor "tramp" (homeless walker) or "hobo" (what we would now call a "migrant" farm worker).