20 x 1 3/8” = 451
Not so.
451 = 20 x 1 1/8", if talking about rim sizes.
Tyres used to be sold by outside diameter, with a fat 20" tyre using a smaller rim than a narrow 20" tyre, so a non-standard tyre would list the rim size it fitted as the size of the standard tyre for tha rim, plus the actual width.
The ISO 451 rim size would originally have been for a 20" x 1 1/8" tyre (20" - 2 1/4" = 17 3/4" = 451 mm), and a 1 3/8 tyre to fit it would be listed as 20 x 1 1/8 x 1 3/8".
Such three-part sizing was a source of just as much confusion as decimal/fractional inches.
Decimal/fractional inches isn't as reliable as it used to be.
It used to be that just 26" MTB (559) was in decimal inch sizes, which made it easy to avoid the other 26" sizes, which all used fractional inches, but size listings have been changing so that most of the common sizes are now mostly decimal inches.
There's still some scope for confusion if not using the ETRTO/ISO sizes though; for example Continental currently list a 28-622 "RIDE Tour" as 28 x 1.1 inches, but a 32-622 as 28 x 1 1/4 inches and a 37-635 as 28 x 1 1/2".