I can only attest to what's on my two Record Aces, and what was on the Raleigh Golden Arrow wheels my Father had on his Paramount - 26 x 1 1/4. Probably a safe guess for Raleighs, but I'll bet more widespread than that.

PS: I see some agreement, but I strongly question whether the Schwinn 26x1 3/8 is equivalent to the British 26 x 1 1/4. I base my question on pretty bitter experience, and a very long walk.