The 645 was the full size, 2nd generation pistol Smith produced primarily for the LE market. The 4506 replaced it.
Pros - Accurate, reliable, easy to maintain.
Cons - Very large, heavy, sights are just OK, grip frame ugly (IMHO), original mags hard to find, sharp edges, hammer bites larger hands, ambi-safety screwed together ( the screw can back out).
I believe the 4506 mags will work. If you have a plastic base pad, you have a 4506 mag. If you have a steel base plate (with or without a rubber bumper) you have a 645 mag. 645 mags work fine in my 4506, but you cannot install the plastic base pad on the 645 mag and expect it to fully seat.
The safety/decocker assembly from a 4506 will fit and correct the screw problem.