Hi Martin,
We're getting away from Johns' original question here but Matchless did use the LW in one of their 500s. They fitted the HW version to their 500ohv V series and the LW version to their 500sv T series machines.
I agree wholeheartedly about using the knowledge of the old guys who have been restoring these bikes for 50 years or more. Unfortunately you describe me and it's depressing to write that, as it means my riding days are closing in. It's frustrating. By the time you really, really know what you're doing your body says, "Nahh, I can't cope with this anymore." A 500 mile ride was once a mere romp, nowadays I just about have to be peeled off the seat after a mere 50 miles. Don't get old!!
And yes, sales literature has to be taken with a grain of salt but most firms did try to get their spare parts catalogues reasonably correct. Regardless of what you think of them spare parts lists are a goldmine of information. Even if the odd bit and piece is fools gold.
Cheers,