Chris,
Might be welded, might be pressed/keyed.
On both my 1934 and 1935 ohv models, the sprocket is tack welded (or tack brazed?) to the brake drum. You should see a small, 1/4 long tack weld about every 2-1/2 inches around the face of the brake drum -sprocket joint. On my 1936 600 Aero, the sprocket was a shrink fit. The drum was knurled for grip and there are three keys around the drum to provide positive drive (see photo). These are pegged through the drum so you should see the a little bit of the 1/8 inch diameter hole one the inside brake surface, as well as the joints for the key & key seats from the back face side.
Unfortunately my original and replacement 50T plate sprocket are temporarily misplaced, or I would have taken a photo of them next to each other. But an older picture previously posted on the forum can be seen here:
Larger ImageNow as to what style the Z/Z1 model used (or tooth count) I do not know. I would expect they were tack welded like the ohv models of the same year. But at some point they changed to the pressed/keyed design of 1936 onward, and that might have started a year or two earlier on the side valve models. I think that less likely of the two scenarios.
Regardless, you are going to have to make your sprocket to suit the style of brake drum that you have acquired.
-Doug