There is only one "factory repair and alteration" place for the m21. That is Connecticut Shotgun Mfg. They bought all of the original equipment, plans, parts and rights to the M21. They do an excellent job with trained M21 people and are the ONLY ones in the US that you can claim a "factory" repair or refurbishment job.
I've had a trap model there for a hinge pin, barrel reblueing and cleaning job done. Very happy with the work