Take a look at the dropouts. The lines are not clean. It's a handmade British frame. It's fine, but it's not the prettiest finishing. Also, the lugs are fairly run-of-the-mill Bocamas or Prugnats. To my eye, it's not special in terms of the 531 frames out there. Is it special to someone else? Well, that's not for me to decide. The fact that it's rare probably just means that it was a small supplier. Frame construction could have even been outsourced to another builder like Woodrup as fietsbob intimated.

I'm not saying it wouldn't ride well. I have a similar 1970s 531 frame with a ham fisted fix on a cracked bb. I rode that for hundreds of miles before I hung it up. It was a lot of fun.