I was just working on a 63-centimeter Raleigh Grand Prix from the late 1970s. That's a 25-inch frame. I have to stand on the balls of my feet to keep certain similarly-titled organs from being painfully crushed by the top tube. Surprisingly on such a tall bike, the top tube measures exactly the same horizontal length as the one on my 53-centimeter Super Course frame from the same period. It's close to the same length as the top tube of my 55-centimeter Trek road frame.
On lower-priced production models, did Raleigh produce only one top tube length, figuring anyone buying at that level would not notice?
I certainly didn't at the time. But I had observed that my Super Course was amazingly long for a bike of its size. I'd never compared it to another Raleigh from the period.
Another utterly unimportant mystery to ponder.