Triton Thrasher:
The frame "fix" was developed by the Norton production engineers in London, not the original N-V engineering design team in Woverhampton. The weakness wasn't fully identified and fixed until a year or so after I left N-V for Boeing.
The AJS motcross team figured it out when the very similar frame used on the factory bikes broke in almost the same way long before the Commando folks became aware of it. We split the top tube along its horizontal axis, opened up the headstock end to position the two semicircular parts close to the top & bottom of the headstock, then welded a long triangular piece into the gap. The seat end of the tube remained circular and the headstock endwas obrund.
The London folks said our fix was too expensive and wasn't suitable for a retrofit.