The .22Mag round was designed in 1959 and in production 1960. Given that the barrel is Chicopee marked I would guess that it was made close to the 1960 date.
That may explain why this one was stamped .22mag. Timing, new product, new caliber, promotional cowboy type stuff.
So, of the 10 alleged Dummy's made;
(assuming all Chicopee)
1) .22 LR
2) .22mag
3)
4)
5)
6)
7)
8)
9)
10)
