This is certainly a rare occurrence. I suspect, that the failure was caused by a bump or strike at the wheel in question some time ago.
Do you have a photo you can show in this forum?
Most likely, the part is identical to the Augusta, the part numbers are the same. Now you want a usable front brake drum as a whole or just the hub which is riveted to the the drum.
I'll look around if I can find any. Or perhaps one of the Augusta/Belna boys attending this forum has one on the shelf?
regards Karl