Not a lot out there on this one, but stumbled across the schematic over on FSB. Apparently it's another Marshall-in-a-box type. Sounds pretty good. Should fit nicely in a 1590B. The 1uF and 2.2uF caps are tantalum in the original but eletrolytics could be used as well. Original 1S1588 clipping diodes could easily be replaced with 1N4148s if need be.