[Arran]

I should add that the double conversion may only be used on the shortwave bands, but I haven't looked at the schematic to confirm. Basically you have two oscillators and two sets of IF transformers, set to different frequencies, it's to improved selectivity, and as MIke said to improve image rejection. Canadian Marconi used to build a lot of radios for military and commercial use, as well as domestic stuff, so they may have done this to create an economy of scale for production purposes. Some also have variable IF bandwidth, but unlike the Rogers sets it was done electrically rather then mechanically, as in the Rogers 10-12. I'm not surprised that the model 86 is heavy, the chassis has 13 tubes, and the cabinet is probably 1'' thick on the sides.
Regards
Arran