Your Uncle's Colt is a Police Positive Special, not a Police Positive.
This is confusing because of how Colt marked the barrels back then.
The Police Positive has a shorter frame and cylinder made for use with obsolete short cartridges like the .32 and .38 S&W.
The Police Positive Special has a longer frame and cylinder for use with longer cartridges like the .38 Special and .32-20.
The same frame was used for all later Colt small "D" frames like the Detective Special, Police Positive Special, Cobra, Diamondback, etc.
A holster made for any Colt "D" frame like the Detective Special or Diamondback will fit correctly, assuming it's made to fit the barrel length of your Colt PPS.
What holster to buy would depend on whether you want a uniformed holster or one that would have been carried out of uniform as many chiefs did back then.
For period type LE holsters there's not much still made today, but El Paso Saddlery makes at least a couple that were used in the 1930's and up into the 1950's that would be similar......
https://epsaddlery.com/
Look at the #5 "Patton", which is very similar to the old "Border Patrol" type holster used up into the 1970's by cops.
For a plainclothes holster for that period, a lot of cops wore originals that look like the "1920 Tom Threepersons".
Since these types of older LE holsters fell out of use, almost no one today still makes them. Several of the last makers went out of business in the last few years, so period LE leather is almost impossible to get.
You might find originals on eBay, but most all of those will be made for medium frame guns that won't fit a small frame like the PPS.