Upvote:4
"I-ТЭ № 681711" includes also the issue series ID - the "I-TЭ" part (which in this case consists of a roman numeral from I to XXXIII and the issue region code (two cyrillic letters), by the way, so it is not "I-T3"). This way, while a single series can't have more than 1 million passports, you can have multiple series, which gives us 33 million passports per region (in 1939 USSR consisted of 138 regions). This ID number is unique only to the document, not to its holder! When the passport is reissued, one is required to register the new ID with all relevant institutions.
Upvote:9
The passport number included two parts, the series number and its proper number. This would allow for enough unique numbers. The passport number though would be changed if passport is re-issued or lost.
The unique identification of a person in the USSR was their name, date and place of birth.
People also had their birth certificate, which was numbered, but again, the number would change if it is lost.
There was no taxpayer number in the USSR, contrary to the answer by Danila Smirnov.