Mea culpa maxima ... the trouble with looking up stuff when one is three parts asleep .... I'll have to lift my game ... thanks, Dave.
And, for the correct regs ...
FAR 23 (at 1964)
http://rgl.faa.gov/Regulatory_and_Gu...A?OpenDocument
FAR 25 (at 1964)
http://rgl.faa.gov/Regulatory_and_Gu...5?OpenDocument
and, for FAR 25, a bit later,
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/701A79191CD71B1286257DC7005718DA?OpenDocument
The reduction from 180lb to 150lb came in at 1978 for the heavies.