I'm sure the list would love the benefit of your insights on why the P/R frame is suitable for 650B but not 650A. Perhaps you could explain the major geometrical parameters of the P/R that were optimized for the 650B rim size, standing, as it does, 3mm shorter than 650A.
Best,
tcs
I think I remember MG talking about being able to use either, but the nicer tires were available in the US in 650B size.
Now, as a practical matter, it may well be that the connoisseurs of fine things bicycle have decided that 590 is too pedestrian and not sufficiently French, so the other 650 it must "B" but that's just a guess, and probably obstructively cynical.