I would guess that, given the era, the patrol officers used a 4" .38 Special revolver of some sort; probably a S&W model 10 or Colt Police Positive. I just Googled and watched a short compilation of clips from the series at this site:
and the gun Lee Marvin was using looked like an old-style 2" Colt Detective Special in .38 Special caliber (hard to tell for sure in those quick, small-frame clips).
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)