That triangular area just under the fuel tank region is also quite short - unlike most vintage frames that had a long flat fuel tank aperture..
That marks it as late 20s very early 30s = early saddle tank design ??
It looks familar, but I can't think what off...
Are there any threaded holes that you can determine if they are cycle thread or metric ?
Don't think its AJS, although there are features that look similar - as did many other makes.