3. Chambering
Even if the receiver doesn’t list the chamber size, the barrel controls it. Since your barrel is marked 2‑3/4 or 3 in., it’s safe for 3‑inch shells as long as that barrel remains installed. Many Express models from this period used those barrels.
I question the above. I've always been told that the receiver must also be marked magnum. What is the reference for this?
Also, in my experience, Rem Police Magnums have always had that (POLICE MAGNUM) on the receiver. That's not to say that all police department shotguns are POLICE MAGNUM models. A few years back I bought 44 870s from a large PD. Some were POLICE MAGNUMS and some were not.