Originally Posted by Tourist in MSN

I do not think that 26 X 1 3/8 is 650C, I generally find data from Sheldon to be spot on.
https://www.sheldonbrown.com/tire-sizing.html

26 X 1 3/8 was a very common tire size where I grew up in Minneapolis, MN in the 60s and 70s, there were a lot of British 3 speed bikes sold in that area with that tire size.

I have a vintage 3 speed in storage that I did not want to throw away, last used in the 1990s. But at that time (pre-internet) I was unable to find the obscure tire size for it for sale anywhere. But now that 650b is a thing again, one of these days I might buy a pair of tires and fix it up.