What is the LOP? Are the chokes so marked on the barrel flats? Have you measured the chokes? Is the forearm numbered to the gun? How exactly is the gun marked 3"?(A photo will help)
Unless you obtain a Factory Letter, you will never know if original or an alteration. Certainly, one could order whatever one wanted from Winchester but there are many Model 21's out there that have been altered for whatever reason.
If Factory, it is indeed a somewhat rare Winchester Model 21 but with only about 30,000 Winchester Model 21's having been manufacrured, one could term them all 'rare'.-Dick