0641 = 064th day of year; March 5 is the 64th day of the year (65th in leap years) + 1 = 1981. Albeit, tons of 81 frames were made in 1980, but it's the assembly date on head badge that is the ticket. Otherwise 1980's have dart on fork while later has the scripted dart.
Not too many 81's with atom drum brakes. Which, for the rear reflector placement in the canti brake bracket, it's highly likely that brake is original. Albeit, pretty easy to stick em on.