Hey Ron, I'm just a clever kinda guy!
I wasn't aware that subsonic ammo had been in use that long.
However, besides the MK I that I examined. I have seen photos of a Beretta Brigadier and a Walther P.4 or P.38 Kurtz with slide locks to prevent any movement. If I can find the book, I will post photos and quote the text.
It is very possible that they would not be required on a Luger considering the stiff action.
Ron