When you say these 531/501 frames are "generic" do you mean that they arrive at the various bike manufacturers already complete for them to finish how they wish or are they just made to a certain template so to speak from pre made tubes, considering they all look pretty much the same throughout. And are those 3 piece forks all identical regardless?
Can you explain it a bit further. What is the score with them all?
Great collection of bikes here btw
I say generic as there are so few differences in the basic layout - I would love to know what they were copied from. Each manufacturer had there own touches such as the wraparound stays on MrK's Overbury or the pencil stays found on others. Some have very neat cable stops whereas some are quite cheap and cheerful.
The tubes I guess, would have come from Reynolds and the used by the individual frame builders.
I was just tracing the bloodlines of the early UK MTB framebuilding scene
As far as I'm aware, the forks are all the same design except where carrier mounts are added. either 501 or 531