With a date of 1921, i would bet it is a mismatch or it is a official rework with a Simson toggle.
I have a 1908 first issue that has many Simson components, to include a Simson toggle.
pictures, especially of the right side of the acceptance and proofs, the bottom of the stock lug (like looking into the magazine well), the full left and full right.
I can look at it and tell you if a true simson or a rework.
need full serial number and if any suffix, you may email me more specific pictures if you desire, but please also post some info here.
ed_tinker@hotmail.com
Sounds like a police that was reworked at some point as the unit marking is Police for sure.
His price if not a Simson is about $400 too high for a 90%, even with a matching mag, but police are much more common w/ matching mags than army.
Ed