You are missing a critical part, a casting, from your jackpot assembly. It is an odd-shaped, ballasted part that prevents the sliding trap door which forms the floor of the reserve to travel far enough to latch on the hasp to the left; it tips backward when coins have filled the JP chamber and allows the sliding trap door to lock shut. I will try to find a picture of it so you can see.
All Dough-Boys were manufactured with a "bug" of sorts: you'll find a deformed cog on the first reel starwheel which, though it doesn't actually make a first-reel bar on the payline impossible, does make it lots less likely.
Nice mouse! I got one just like it with my Dictator.