well im not really sure but i would tend to say they are legal as you could make and buy them back then and you can make them now. why should they have to be a vintage brand name when any person with the machines could have knocked up a set in their back shed in 1978 or what ever? they are just a machined block of aluminium anyway. if i do travel down this route and they are illegal, i dont really care as my bike is not going to be raced at vintage MX events anyway.
personally i would be a little disapointed if they were illegal when all sorts of modern shocks are allowed, but as i said before its not really an issue for me.
i would prefer to actually find some White Brothers or Profab triple from back in the day but cant find any, but by all means if you have some you want to sell let me know, so next best thing is to just make some, which would be better than the WB/Profab ones anyway as they had a flimsy top clamp.
i think Lozzas idea is good, as much as i would like some from Kelvin as he has been making that stuff since 70's/80's, his price is a little high when i could save some money on the CAD drawing.
the machined bare alloy clamps are stronger and look much nicer than the painted cast alloy originals found on most bikes.