That's true Andy. And vintage amp aficionados pay a fortune for the RS trannies that were used in the first JTM45s. Even RS catalogues from the early 60s era fetch good money as guys like to check out and ID the components used in Marshall amps. There's a whole world out there of 'home brewed' JTM45 amps, fuelled by guys producing trannies, cabinets, control plates fittings, etc etc. In fact I am in the process of building a '63/64 version myself. The cabinet is being made in the US.
Here's an example:
http://www.merrenaudio.com/marshall_output_transformers