I’m no expert on vintage tire sizes, but here is how I understand it.

26” refers to the outside diameter of the tire. The actual beam/rim diameter can be anywhere from 22” to 23-1/2” (559mm to 597mm)

27” tires also refer to the outside diameter of the tire. I believe the standard beam/rim diameter was around 24-3/4” or 630mm.

In the British Empire and it’s former colony across the pond, the Imperial system of inches flourished for many years. It is so ingrained that people ride 29ers with the same bead diameter as a European road bike (622mm).

Meanwhile the French, and I suppose other countries, have a size called 650. It’s diameter was 650mm outside diameter or 25-1/2”. There is an alphabet soup of bead diameters that ranged from 22-1/2” to 23-7/16 (571mm to 590mm).

So yes 650b is historically a 26” tire, except for mountain bikers. No one will buy a 26+ mtb, so the logic is a bigger modern mountain bike tire is 27.5” in diameter. Likewise a 29” outside diameter tire can’t be called a 700mm outside diameter because it is so much beefier. But we won’t talk about 700c X 40 tires.

Such is life.

John

Edit Added: 700mm tires have been referred to as 28” tires using the rounding system from 27-1/2”. In retrospect 28” might be more accurate these days than 700mm.