Made in Great Britain they are heavy with lots of real iron. The amps they were removed from utilized a CRC filter / dropping network to set the voltage at 5VDC for the 300B tubes. These were removed from a custom-made pair of SJS Monoblock's that used a 300B final and a WE 417A (5842) input stage.
I have provided a picture of the Amps schematics. If you have any questions, please feel free to ask, my name is Mark.