26 x 1 3/8" (590mm) was a popular British size BITD, especially for 3 speeds, AKA 650a.
Raleighs and others used 26 x 1 1/4" too (597mm).
French bikes I think used 650b size (584mm).
MTB standard 26" is 559mm.
Then there's 650c = 571mm, old skool triathlon machines mainly IIRC
Hope this helps - might narrow the search a little and prevent incorrect tyre purchases?
PARTS FOR SALE:
vintage Brooks Professional saddle | Specialized FACT CX fork | Columbus fork | Reynolds 531 fork | Avid Arch Rival v-brakes | various 700c, 27" and 26" wheels | vintage Weco hubs (fixed/freewheel) | vintage Raleigh ladies frames + lots more!