I, too, carried one as a duty rig for a while in the late 70's/early 80's (Safety Speed from when they were in Montebello). I used a 6" version with a Model 19 and then a 4" version for a Python.
They flopped around enough that you could never be quite certain exactly where your gun might be...
The holster/gun weight and constant motion often caused the swivel to wear through (equalling your gun, still in the holster, on the ground)...
The interior was horrible on the guns; they were not a snug fit and there was enough exposed metal rubbing on the gun which did a lot of damage...
Re-holstering your gun was a pain-it typically required two hands and you had to be sure it snapped shut completely...
They would require tweaking to ensure that the mechanism remained "just right;" not too easy to open, not too difficult.
Not a safe holster, but a very comfortable rig that was super fast.
I sold mine to a friend who said he wanted them for a collection of ancient police stuff. I should have raised the price or punched him for the comment.....
ETA: Here you go, an LAPD-spec holster for a 6" K-frame
http://cgi.ebay.com/SAFETY-SPEED-HO...tem&pt=LH_DefaultDomain_0&hash=item19bc81b9b8