That MUST'VE been our very own Bren photo maestro, Gev G. Look through his old posts and I'm sure you'll find it.

What is a Mk3/1 by the way? Or do you mean an Indian Army lightweight Mk1 designated the Mk 1/3? Whatever it is, avoid the Mk1 gas cylinder like the plague! Mind you, if it's for occasional use and cleaned properly after shoots and at least every day for a week afterwards it should be OK for ages