Any question answered...

If you go back to the earliest café racers,it probably was the best material available. Now it's there for the hipster points :-)

That is what I was thinking, looking at the ducati's they just use cro mo

531 might have been the best material available, but it wasn't used for cafe racers. The earliest were cobbled together with proprietary frames.
Clipons, rearsets,engine swaps alloy tanks and single seats are what make the earliest cafe racers.

Unless you were lucky enough to come by one of the race team Featherbeds :-)

Manx norton?