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:
http://www.timelessvideo.com/dvds/msquad.html
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).

Chicago issued the Model 10 for many years ( maybe issued isn't correct, but approved). However it was usually considered a backup gun due to their very broad second gun policy.