Its highly unlikely anyone would spec a bike with 531 forks unless the rest of the frame was 531, as the biggest weight saving comes from the main tubes, and having at least the main tubes in 531 has got to be a better selling point than building a hi-tensile frame then just fitting 531 forks only.
I think the CB Cresta had 531 forks only: http://www.nkilgariff.com/CBcats/Cat_77late/CB_Intro.htm