I don't have enough info to know for sure, but if you look at the Sunn schematics dated 10/70, they still have the 5AR4 rectifiers. Then the '72 Catalog gives power ratings of 80W and 150W to the same amp circuits which were previously rated at 60W and 120W, respectively, indicating the implementation of SS rectification. So the change probably occurred during or at the end of the 1971 production year, if you figure that the '72 catalog was edited in late '71/early '72.
Anybody have additional info on that?