Hi Kev...Not too sure about that...The M20 stand is tubular and fits either side of the frame (the forging on the upper end of each leg forms a 'clevis'.) The stand bolts pass through one side of the clevis, through the frame and are screwed into the other side of the clevis. So the stand bolts pivot in the frame.
On a B29 I owned briefly the rear stand was made of brazed together forgings...not tubes and the attachment points were not in the form of a clevis but were single sided, just fitting against the outside of the frame and pivoting on the stand bolts, which were screwed into the frame.. The military WB30 I recently restored had a tubular version of the B29 type. The M20 stand is also wider and has a pronounced kink in one leg.
I think the M20 stand could be made to fit but you could say that of virtually any rear stand...A Triumph stand is actually closer to the B29 type which would be either the same or very similar to the B26 one I would have thought.
Leon, who comes on this forum may have some comment on the subject as he is a 1930s BSA collector....Ian
email (option): ian@wright52.plus.com