If that is indeed a downward sliding mag release, that I think it would be something of a new idea. Well, HK has levers at the very corner of the trigger guard, but this HiperMystery pistol seems to have a sliding button (really a lever which stays flush against the grip) moved back on the grip. I can't recall any other pistol which has that sort of thing in that location.
And this is enough to make me pay attention on 31 May. If I don't forget in 6 days. That seems to happen to me.
Bart Noir