[CR]Help dating a BSA track frameset

I seem to have a thing for certain British brands. With cars it was Triumph and now with bikes it's BSA. Anyways, I recently picked up another old BSA frameset that seems to be very well constructed (particularly with regards to the thinned lugs) and could use some help in dating it. Unfortunately, a prior owner refinished the frameset with a poor rattle-can paintjob and the original markings are no longer known.

The serial number stamped on the seat tube looks like it's 2101, and some photos can be seen here:

http://www.chainedrevolution.com/images/bikes/bsa/details.htm

Thanks in advance for any comments offered,
Tam Pham
Huntington Beach, CA