the front end doesnt suit the frame ... i think its almost certainly a royal enfield front end and wheel
the silencer is a goldstar silencer
strange carbs - not original
i think the engine looks like the later a7; the engine that was a scaled-down a10. Looks like a twin-carb head; The SS had a twin-carb alloy head, so i dont think its an SS ... the Star Twin had a twin-carb iron head, so probably a ST head.
The basic engine nos are:
1950 - ZA7 (Star Twin - ZA7S)
1951-52 - AA7 (ST - AA7S)
1953-54 - BA7 (ST - BA7S)
1955-61 - CA7 (Shooting Star - CA7SS)
In 1961 there was an alternator version - CA7A
If you want a full number list try Draganfly Motorcycles
http://www.draganfly.co.uk/bsa_tech.htmljust noticed the date this was originally posted; oh well