I believe rotary convertors were a motor rotating the shaft of a dynamo. That is I am sure what was use to power the military 19 sets.
They were quite powerful devices, I was able to use a Wolf electric drill on the DC 200 Volt (?) HT supply from the rotary convertor.
Yes, it was mounted in a separate box which sat on the left of the 19 set, with a big red light on it and a massive multi-way plug linking the two. I remember someone mounting one in the engine compartment, used to power his electric razor whilst camping.
I've converted 19 sets and R1155's from Lancasters in an earlier life.