The 55 volt regulator employs three transistors, VT1 the error amplifier and driver, a 2N698. The two parallel connected series regulators VT2 and 3 are a Texas type 2SO34. There doesn't appear to much information on this type, a type NT130 is said to be a near equivalent. I suspect the popular 2N3055 would serve as a replacement. The 2N3055 is one of those components that seems to be have been around for ever, the Radiomuseum suggests since 1960. I didn't think it was that early, I would have thought it was about 1964.http://www.radiomuseum.org/tubes/tube_2n3055.html
Anyway, the 55volt regulator fault turned out to be an easy fix. The wire going to the collector of one of the 2SO34 transistors had become disconnected. The picture is now at full height again.