I have a second-party (not Reynolds) listing of available tube sets from 1975. It lists "531 Club Standard", "531 Racing" and "531 Special Record", but no SL. The Special Record is listed at 1795g., butted, for record attempts.
As an aside, I remember in the early 80s, the Cycles Peugeot sales rep coming into the shop and offering me one of the custom Peugeot 531SL frames that John Marino used in the RAAM (or whatever it was called at that time...Marino challenge?) I passed, but they exist.
Ed Kasper
Detroit MI