Think in prewar times the tubelar type stands were only fitted to the military contract M-types.
The civilian (and some military) prewar M models were usally fitted with a forged steel one, which was originally designed for the frame and fits easy on the outside on the frame.
Always wondered why BSA choose to switch to the tubelar forked ended one we all know for which there is actually no room on the drive side.... Did the WD ask for a stronger/cheaper one ??
Interestingly, they did the same thing on the WB30...The B29 Silver Sports, on which it was based, had a forged stand but the WB30 had a tubular one...
In that case the tubular one was much stronger...Ian