Through the Patina, that number looks like ZA7S 428. It started life as a Star Twin, 1949. It is early in the production schedule, the Model Year started at ZA7S 101. HC is a High Compression (for the time) variant.
Cylinder barrel flange looks to be Longstroke Type, so I hope this to your liking and you have not bought a lemon.
Picture has worked fine, give us a bit of a gallery. Just have another look at the frame number. Could be a later frame with an earlier engine. Have you any paperwork with this purchase?
Get spraying with the WD while you decide your next move, and I reckon you will need a bit of heat on those nuts!
Swarfy.