I'm happy to engage in a bit of 'artistic license' as I don't really build anything to a full 'rivet counter' specification myself...
Practical issues apply sometimes as well.
For example, the webbing 'ring piece' seen on many WD bikes these days to prevent marking of the petrol tank paintwork evolved into its current neat form based on a single photo I saw, a number of years back, of a very roughly cut piece of webbing applied to a Norton tank. That seemed like a good idea to me so I tidied it up and fitted one to solve a problem...It seems others agreed... (if only I'd bothered to copy that picture!)...Not historically accurate though, or widely used it seems. Just another rider modification perhaps..
Canvas saddle covers are another thing that comes to mind...I think we concluded after much discussion they were intended for a self propelled gun, not a motorcycle...Now, that's not to say one was never fitted though... I've seen them fitted currently...
Green canvas bags...Late war or postwar?...A hard one to prove definitively...There are in fact numerous examples along these lines...
Regarding air filters it's clear the WD experimented and it's equally clear REME used a wide range of filters 'in theatre' to mitigate an obvious problem...
Despite all the rules, circulars, parts lists etc. etc. it seems nothing was cast in stone once the bikes went into service and who could argue against using 'non standard' fitments recorded on photographs in theatre?...(Like the Canadians liberal use of foam rubber seat modifications)...
Today some want a completely correct bike in every way and others aren't really concerned beyond the general look of a WD machine...The latter is fertile ground for 'specification creep'..
The point of my (light hearted) comment was only to illustrate the danger of 'accepted practice' or if you prefer 'artistic license' becoming regarded as the original specification when commonly seen...Many desert M20s are presented today with full Vokes equipment fitted, including the 'cut off' petrol tank..That is not historically possible...In fact I fitted one to mine when I finished it in a 'desert' colour because I incorrectly thought at the time it was supposed to have it...
Is it a true statement that when it comes to original specification the only quick reference that can be relied upon in most cases, without extensive research, is the manufacturers parts list?...Ian
