Then there's this!

I really had no idea that there were so many totally different '26-inch' bicycle tires! I grew up riding 26x1-3/8 - which I now know was the 'non-Schwinn' ISO 590 E.A.3 size, not to be confused with the 26x1-3/8 ISO 597 Schwinn S-6 size, or the ISO 559 decimal size used on MTB and Cruisers... Lately, there has been a resurgence of ISO 584 650B which is also known as 26x1-1/2 or "27.5"...

https://www.sheldonbrown.com/26.html