T or not, the first 2 numbers are the year of production. as it was produced to the special needs of the kgb, there were variants without numbers and other specialities. I own one and I have a slicer to cut film for it. it works well. http://www.subcompactcam.com/21mm_ajax_f21.htm
As xya said the first two digits represents the year, but it's from an unknown date on. Very early F-21 had different numberings I think (it's like the early Zorki-1 series before 1955). The "T" models are the newer ones and there exist special versions like the one for artic use for example.
An extra version are the 1994 F-21 with no s/n on the front but on the back. This is the last year of production as far as I remember. Also in this years the civil version MF-1 was build by KMZ with the typical KMZ logo and the s/n on the back too.