Fastback points

I believe that the clue to this should be in the engine number. The later, points in timing case, engines have numbers starting 20/M3S. Which appeared late in 1969. Probably when the factory moved engine assembly to Wolverhampton. Before that date, the engine was based on the old Atlas model. Atlas bikes had engines with numbers starting20/******P. The next generation, used in the Commando have numbers beginning with just 20/M3/******

1969 was the big changeover year with the Comando having arrived on the production line, the odd export Atlas and650SSstill being built and the Mercury also appearing for sale. Plus the factory move.