I have both lewis leather 2" and 4" and 6" clamshell. I did many years LAPDI have never had one, but know what you mean. It was almost like a backwards Berns-Martin break-front, made so you drew it out the split back. Sorta like the Bucheimer-Clark, with the spring coming up and behind the cylinder instead of over it like the Berns-Martin..
The later Dirty Harry shoulder holster looked like it was basically a Lewis converted to shoulder use.
I guess LAPD liked trick holsters back then, what with the clamshell-type on Adam-12 that spring open when you hit the button by reaching through the trigger guard (shudder). At least two places made those that I know of- Audley and Safety Speed Holster- so there must have been a market.
For some reason.
Although, as a holster junkie, I do want one for a conversation piece.