It was all about the "specialties" to folks in various areas of the country; still is, in fact, even for the soulless, far-above-in-numbers-of-every-other-service-pistol Grocks. Clamshell and clamshell-type uniform revolver holsters were still popular in the west coast well into the 80s. In the southern and central part of the country, clams were not well-liked by most; by far the solid holsters with various thumb-straps were popular.
And then of course there was that Jay-Pee (?) thing that lots of eastern cops used back then, along with other, similar types.
You know what disturbs me these days? Many of the plastic/fake leather service holsters most cops are issued these days have a lot of second-and-third type of "safety" devices, that makes me wonder just how easily- much less quickly- a lot of those things are at getting out service pieces. Of course, part of the problem is that most of the new hires really don't know that much about guns in general, nor do they care. Most of them do the best they can in regards to what they are issued and taught with, so I suppose things could be worse.
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