B31.1 Allowable Stress Basis

Earlier the allowable stress basis on B31.1 was based on 1/4 of specified min. tensile strength or 2/3 of yield stress, whichever is smaller. (ignoring creep criteria)

It has changed to 1/3.5 of specified min. tensile strength or 2/3 of yield stress, whichever is smaller.

On what edition of the code this was effective from?

If I remember well, it was in the 2005 Addenda to ASME B31.1 based on Case 173-1 and following the changes implemented in 1999 Addenda of ASME Section II Part D.