H&R Model 732 - (early production called "Guardsman") introduced in 1959 - 1st Model (live firing pin in frame, later on hammer) ran 'til 1973 - when H&R introduced a hammer block safety and ignition system to their hand guns and at that time it became the 2nd Model - which remained in production up to 1986. Originally offered in 2.5" barrel with short rounded butt and 4" barrel with squared butt and drift adjustable sight. Most of later production was of the short barrelled variety and 4"ers are scarce. Your piece with 'AP' serial prefix was produced in 1976, which would make it a 2nd Model. This model was H&R's first center fire revolver with a swing-out cylinder and followed the pattern of the M929 rimfire - which was H&R's first swing-out cylinder revolver.
For informational purposes, here's pictures of the H&R R&D PROTOTYPE of the Model 732, this was put together on a cut down Model 929 frame:
and here's a pic of a very early PRODUCTION piece w/ short barrel and full size square grip frame:
Here's a pic of the 2nd Model: