Maybe its off a single??? Some of this early BSA stuff is baffling. So many variations of basically the same parts.
I am sorting early BSA parts and later unit singles and twins and its a game of "Trivia Pursuit" For example, sold a 67 Hornet, The wheels were off it but I had them but along with other parts had been intermingled when we closed my shop and then moved the inventory 3x. I KNEW I had a ton of BSA parts but seats?? Wheels? Axles? Oil tanks? All the same right? Nope!
It took most of a day and my pile'o axles did not produce the needed one, but finally found the correct one for the rear wheel, I dont know how many variables there are, but discovered that the competition models uses a slightly different rear axle than the road going ones.