Updates from the comments
First: From Somer “That is a Scrambler (very good point and on second glance I agree .ed). Flat trackers don’t use brakes. Probably an iron barrel B-31 or B-33 BSA Pre 1954.”
At first glance it looks like a B31[350cc] or B33 [500cc] motor in a standard swing arm frame.
This engine had alloy crankcase and cast iron barrel etc..it was the standard ride to work BSA here in the UK. Production stopped about 1960 although back stock sales continued to 61/62.
From this was developed the Gold Star series.. B32 [350cc] and B34 [500cc] and eternal glory.
Well, for now anyway
Front brake is standard [tho’ not for this bike] late 60′s Triumph/BSA 2ls stopper.. good too.
The tank and seat are sooo lovely.. late 50′s/early 60′s moto X. Beautiful.
Black frame would have been nice.. as original.
Hope that helps a bit.. maybe someone else who knows the bike can fill in more.
I could use a little help on the ID specifics if anyone wants to chime in.