I'm not sure if there are scallops in the top of fuel tank?
Ariel used them as something of a trade mark. Twin port singles were relatively common through the 1920's
My father had a 1928 BSA Sloper (now passed on to my brother) It has relatively small, almost round knee pads on tank
It had nickel plated parts, most of the bikes were painted as it was cheaper (depression years) so plating was optional
The BSA girder forks were distinctive but I don't remember why (my father died a couple of years ago so cant ask him)
PJ