I like the movie blank prop gun idea. TTs are very popular in asia, one of my favorite action films "Hard Boiled" shows many examples of TTs shooting blanks including a nickled one with a suppresser. That mag looks about right for the 7.62x25 blanks that I've seen in the past. Another remote theoretical possibility is 9x18 mak since it would be a logical first conversion idea in com block countries, though I've never seen such a thing.
Every day is an adventure of one sort or another, if you live, you learn.