Aut cum scuto aut in scuto
Possibly a T-150 or T-160. Most likely a T-160, as that model had the sloping cylinders ala the A-75 BSA.Most twins had a riveted patent plate. The triples had screws to access the timing marks, as the alternator rotor is on the right side, below the points cover which can be seen above right. I owned a T-150 years ago, and it is still alive and well, burning the roads up in Wisconsin.
You didn't show the tag lower down which stated that "All parts falling off the machine are of genuine British manufacture".