Hobbs Frame Numbers.
Hobbs frame numbers are on the rear dropout and will begin with a letter followed by either three or four numerals.
The letter and first numeral are most relevant. The number on the BB is not the frame number.
One of my (three) Hobbs frames is D1***, so D = April, 1 = 1951, this tallies as my Wife's Dad bought it in the summer of 1951 to make a tour of the Loire Valley.
Hobbs stopped building frames in 1953. Hope this helps...
Roadking.