When I started at the Houston Police Academy in 1983, the recommended revolvers were the four S&W L-Frame models. Several K-Frames were also allowed, but the memories of the early Model 66 cylinders expanding excessively, due the heat during firing, were still fresh, so blued-steel was recommended. The only Colt that was approved for a cadet was the Python, and one Ruger Six was also OK; I have forgotten which one. All had to be chambered for .357 Magnum, and all had to have 4" barrels, but longer barrels were OK to use after graduation from the academy. Then, as now, HPD officers have been expected to buy/provide their own firearms. I bought an S&W 686 for duty, a 581 for a spare, a 66 for personal-time carry, and had previously bought a 60 for back-up. (Sadly, I kept none of these, mainly due to a big sell-off during lean times, about 1986-1987.)
After graduation from the academy, duty revolvers could have any barrel length, 3" and longer, and could be any cartridge from .38 Special to any of the several .45 cartridges. (A wide range of autos were OK after one year of wearing the badge.)
The then-trendy "man's gun" among revolvers was an S&W Model 29 or 629. I think one reason my right* wrist is now a train wreck is because I just had to be a real man, and carry a 629 while a rookie. (In order for my medium-length index finger to reach the trigger, I had to use skimpy-sized grips, which did nothing to mitigate the effects of recoil.) In 1985, I backed-down to a .41 Magnum Model 58. About 1990, I started carrying a Colt Commander, then switched to a SIG P220 that I carried from 1991-1993, and returned to revolvers in 1993, carrying a GP100 and S&W Models 19 and 66. In 1997, I returned to autos for duty, permanently, but have never totally abandoned revolvers for back-up and personal-time carry.
In 1987, in an move apparently calculated to improve the department's image, revolver barrel lengths over 6" were no longer allowed, and barrels longer than 4", but not over 6", were grandfathered. All newly-added revolvers, for carry on or off the clock, since 1987, have a 4" barrel length limit. (Yes, PD rules apply 24/7.)
Some time in the mid-Nineties, the cadets were switched to starting with several specified .40 autos, and by late 1997, all newly-added duty pistols, had to be had to be three specified .40 autos. Our existing duty autos and revolvers were grandfathered.
*Although left-handed, I am reasonably ambidextrous, and a long-stroke DA trigger is caveman-simple, anyway, so I started carrying "primary" on the right hip, and have continued to so do. An unintended long-term positive consequence is that my more-skilled left hand was largely spared the pounding of bore-bore Magnum recoil.