OK, here are some photos.
As I mentioned before, the sideplates on these guns are a precise interference fit in the frame. The sideplate will not lift off when you remove the two sideplate screws.
DO NOT TRY TO PRY THE SIDEPLATE OFF!!! Prying it off will result in damage to the sideplate and/or the frame. The sideplate is ready to be removed in this first photo. Notice I have sketched a dotted line across the frame.
Hold the gun in your left hand, with your thumb lightly resting on the sideplate and latch. You are going to strike the frame of the gun right where I drew the line. The wooden handle of a hammer is a good thing to use. Do not use the metal end. Holding the gun as I described, strike the frame with the hammer handle at the line. Inertia will keep the sideplate from moving while the rest of the gun is whacked down. You will feel the sideplate start to lift with your thumb. Keep striking the frame until the sideplate pops free. Keep your thumb on the latch the whole time. There is a spring under the latch that will want to fly free and your thumb keeps everything from flying. Remove the sideplate with your thumb still on the latch. You can now place the sideplate and latch on the bench and carefully remove the latch and spring.
Here is a photo of the backside of the sideplate. The latch is still in position and you can see its spring through a slot in the sideplate. Notice all the gunk.
In this photo the latch and its spring have been separated. Really no big deal if you control the latch while you lift out the sideplate. The spring fits into a horizontal hole in the sideplate and there is a small relief in the side of the latch that receives the other end of the spring.
Here is the gun with the sideplate removed and the action ready for further assembly. This Police Positive was made in 1932. I have had it for about ten years or so. It has always been a bit sluggish, so this was a good opportunity for me to go in and clean out all the hardened old oil and relube it lightly with Rem Oil. So thanks for the excuse to take it apart.
For those who are afraid to get under the sideplate of an old Colt or Smith, I certainly understand. When I was just a kid in the 1970s I took the sideplate off my Model 17. What I saw scared the dickens out of me and I quickly put it back on. I had taken my C&B revolvers completely down, but all those parts inside the Smith scared the dickens out of me. I did not get up the nerve to get inside a Smith or Colt DA for many years, until I had some expert guidance by a friend, and had the Kuhnhausen books in front of me.
So excuse me for now, I have to go back down stairs and finish cleaning up that old Police Positive.