I have a '57 ? with a frame # 15225 and motor # M23658B , so yours is a bit earlier than mine . Maybe a '55 ?
I don't think they started numbering the frames until about 1950 .
Mike Sol keeps a registry and would likely have a better idea ...
The shut off screw has a packing to keep it from leaking . You'll want to empty the tank before you remove the screw . But you might try tightening the compression nut around that screw first .
The rack looks like it's from a bicycle .
I can understand that Reese .