I'm thinking pre 1940. Agree with the guys above, mono-blok, rib, etc. indicates early gun and matches my book. Looks like a single non-selective trigger available at that timeframe. That rear lower pin would place it closer to 1940, but the monoblok style says it's earlier. The Midas pictured in the book which was delivered in 1947 shows a triangle shape with a gold eagle(?) in it and sparse gold wire filled scroll and geometric shapes on a blued frame. So, probably not a Midas.
I'm thinking an early development of the Diana or Grade 5/6 possibly early '30s.
