SaxonPig: I think you've got your decades wrong. Revolvers were not only carried by older cops in the 1960s and 1970s, they were issued to and/or purchased by younger cops. A lot of agencies did not even permit the on-duty carry of semi-automatic pistols. I would bet the majority of off-duty guns (if the issue gun wasn't the off-duty gun, as well) were revolvers, as well. I remember an awful lot of 2.5-inch barreled M19s and snubbie S&W J-frames and Colt D-frames. One agency I recall required a special qualification for officers who wanted to carry an automatic off-duty (BUGs were officially taboo and would earn you a two-week, unpaid vacation if discovered). Those were most often variations on the Colt M1911 (Government Model, Commander, Combat Commander) or Browning Hi-Powers with some .380 Walther and Mauser pistols mixed in.
Of the major agencies with which I am familiar, only Dallas PD, Houston PD and Harris County SO allowed the on-duty carry of automatics and that was because the officers and deputies purchased their own sidearms through the department. And even then, the majority carried wheelguns. Austin and San Antonio required, or issued, revolvers well into the 1970s. I know Austin was issuing M27s until the early 1980s.