It's essential knowledge if you want to date documents, and especially if you want to check whether the dating of a cover is plausible in relation to the periods Before Biros (BB) and After Biros (AB) The cover below from a recent auction is definitely AB which is also the historical period of the envelope used, a 1960s (plus or minus) Soviet-style Mongolia Air Mail envelope. I can't read the date on the cancellation but you can be sure it isn't 1933 which is the date assigned by the auction house. It's also not Ulan Bator, is it, also the auction house's claim. I have once before seen these 1932 Mongolian stamps used late, like this. What I don't know is whether post offices started using them again at some point or whether Soviet-era East German technicians in Mongolia spiced up their letters home with philatelic frankings
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please note the postcode. In Germany we used the prefixes "O" and "W" for East Germany and West Germany only between 1990 and 1993.
The postmark date reads 93-3-10. So the postmarks are from 1993.