Hi Eddie, after much browsing of the literature and photo libraries I have decided that while "un-modifying" the MKIIIS was the initial goal, there are a few period / sympathetic custom modifications I'd like to do. The base machine was modified with MkIV/V components over 50 years ago.
I am restoring the colour scheme, single leather seat, up & over exhausts and long silencers, however, I'm also looking to change unnecessarily heavy &/or less aesthetically pleasing features for period alternatives.
These include the steel mudguards which are not in very good condition - the rear has been cut and shortened. Alloy wheel rims, which I believe were a factory option, if not in 1948, within a year or two.
I also plan to switch to flat handlebars, similar to the earlier T35.
Motives are partially cost and time constraints too. Re-chroming the original bars and wheel rims is expensive in the USA, quality restoration and refinishing of wheels and mudguards exceeds the price of very similar aluminium aftermarket ones.
I think I have found a suitable 5.5" wide rear mudguard, which can be extended at the front with a straight simple curved piece to pick up the lower mounting lug and re-formed to clear the chain on the left.
Measurements of mudguards can be deceptive though... you see the dimensions and think "eureka!" then offer it up and find the curvature is just wrong enough to make it look very odd!