I am interested if you decide to sell it. I am assuming it will work in a 21.
With the .45 bolt and recoil spring assemblies removed, the 21 and 28 are precisely identical.
So, yes, the kits work perfectly in both.
Also works fine in the M1, if you bought the complete kit, with the M1 cocking knob and muzzle nut.
Biggest difference in firing:
The Merle kits have a steel plate on the aluminum bolt bottom to slow the ROF to about normal .45 speed.
This also means the sear notch will hold up about forever (which is also approximately how long a .22RF barrel will last).
EDIT: See post no. 20 below. Not all sear notches...
The jac bolts don't have this plate, and fire at a much higher rate, often over 1000 RPM.
Merle's told me his first venture into .22 conversions actually started with just adding the plates to jac bolts.
Edited by mnshooter, 22 May 2019 - 10:19 PM.