Soon after the ISP adopted the 39, the Chicago PD placed it on the approved carry list for off duty and BUG. However the order required to weapon to be carried with the "hammer down on an empty chamber", same as for the 1911 and BHP. Needless to say that portion of the order was honored more in the breach than the observance. Many detectives and plainclothes officers carried it as a primary, and left their "legit" service weapon in their lockers, or more prudently, packed a M36, or later an M60, or one of the LW J Frames.
There were some NDs, actually quite a few-with revolvers and shotguns too. Even today if you walk into a Chicago PD facility you may see photo of the mayor or police superintendent in a peculiar place, like high up in a corner or some such, it is probably covering a bullet hole.
Actually if I recall correctly(and I do) the CPD had one of the lowest hit rates in the US. If a CPD officer fired his weapon intentionally he hit the intended target less than 16 percent of the time, if he had an ND, he hit someone slightly over 18 percent of the time. Go wonder.............