Had a very helpful - and very prompt - reply from Ashly to my email. The schematic above is for the early version of the SC-50 which uses matched pairs of transistors in a metal can. This is the type I have. The later version uses a dbx chip
According to Jim at Ashly: "If you do have the original version SC-50 with the matched dual can transistor VCA, it is important that no substitutions be made concerning the IC chips. The circuit was designed around a special mode of operation only available in the 301 IC"
Thanks for the help, shot & Jim
Nick Froome