Do you mean the mid-'90s Police Positive Mark V? I believe they were all 4". The previous late '70s issue with the half length barrel ejector rod shroud were all 4" as well...or at least I've never heard of anything but 4" for the PP and the aluminum frame Viper.
Any other barrel lengths were Detective Specials, Cobras and Commando Specials (1980s).
By the 1950's the Police Positive Special was only available as a 4 inch standard production and "possibly" as a 5 inch on special order.
The 2" and 3" inch Colt "D" frames were the Detective Special, Cobra, and Agent.
In the last year or so of production the Police Positive Special was stamped as "Police Positive MKV".