Posted per request from a forum member, from M14 Rifle History and Development Fifth Edition:
M14 - Service grade rifle assembled in 1991 and 1992 only with Chinese parts
M14 R - Service grade rifle assembled with USGI parts and USGI synthetic stock. This model was available from 1986 to 2006.
M14 RNS - Service grade rifle assembled with USGI parts and National Match walnut stock.
M14 RNSB - Service grade rifle assembled with USGI parts and USGI birch stock.
M14 Beginning National Match - Match grade rifle assembled with hand fitted USGI parts and USGI National Match barrel from 1993 to 1996.
M14 NMR - Match grade rifle built to the USAMTU specifications with National Match sights and a choice of standard, medium weight or heavyweight barrels. This model was available from 1987 to 2006.
M21 - Match grade rifle with a rear lugged receiver and choice of McMillan fiberglass stock or a wood laminate stock. Rear lugged match grade rifles were available from Armscorp from 1986 to 2006.