Around the turn of the last Century Colt introduced their small framed revolver, which they called the Police Positive.
This was an upgrade from an earlier pistol, and the "positive" referred to Colt's new "Positive Safety" system that prevented the gun from firing if dropped.
The first PP models were chambered for lower power cartridges like the short .32 and .38 cartridges made by S&W and Colt.
In 1908, Colt lengthened the cylinder and frame to accommodate the new .38 Special cartridge, and this was named the Colt Police Positive Special.
The PPS was offered in a wide variety of cartridges like the 32-20, the .38 Long Colt, .38 Short Colt, and the .38 Special.
Fairly quickly, Colt offered target versions of the PPS usually known as the Police Positive Target models.
The PPS was quite popular with many police departments, and civilian shooters.
After WWII, versions of the Police Positive Special were made in smaller numbers up into the 1980's.
If you'd like information on YOUR particular gun for heirloom purposes, you can get a Colt Historical Letter from the Colt factory.
This letter costs about $100.00, and contains everything Colt knows about the gun.
This would include the original finish, barrel length, any non-standard custom options like possible the ivory grips, and when it was shipped, and to who it was shipped from the factory.
These Historical Letters are made of document-grade paper, and can be framed.
http://www.colt.com