Thanks vpl -
I tried searching the Federal Register but I didn't have any luck. The FR search engine isn't that good (or maybe I just don't know how to make it sing). I'm not sure if changes to SRPs (NUREG 0800) appear in the FR, so I didn't try that approach too hard.
The original BTP was from the Reactor Systems branch, and appeared in SRP 5.4.7. Also, it (BTP RSB 5-1) appears to have replaced (? not sure about that) Reg Guide 1.139 (which had been issued for comment in May 1978).
Thanks again
gmax