First is it a Colt Police Positive, or a Police Positive Special.
These are two different guns.
The Police Positive was made for short, obsolete cartridges like the .32 and .38 S&W.
The Police Positive Special has a longer cylinder and frame for use with longer, more powerful cartridges like the .38 Special.
Next, you need to re-check the serial number. If the "R" is stamped below the serial numbers it's NOT part of the serial.
Letters stamped below or "around" the serial number are inspectors stamps.
Only Colt revolvers made after 1966 had letters as part of the serial number
With that said, if the gun is a Police Positive in .38 it was made in 1908.
If it's a Police Positive Special in .38 Special it was made in 1910.
If it's a Police Positive Special and the serial number is 22513R, it was made in 1977 or 1978.
Value depends on the amount of actual factory finish remaining.
Assuming it's a 1977-78 PPS and is in at least 95% original condition the value would be worth more than $450, as a guess. Colt values are going up so fast, it's hard to keep up with them.