It sounds to me like you might
really enjoy the book, Orchard, C. J.; Madden S.J.
British Forces Motorcycles 1925 - 45. Sutton Publishing Ltd. (2006 rev. ed.) [ISBN
978-0-7509-1445-1]. My hunch is you'll want to
buy a physical copy for later ready reference for your project. But, at least for now, you can catch a freebie cover-to-cover scan of it online care of Scribd at
https://www.scribd.com/doc/38290548/Sutton-Publishing-British-Forces-Motorcycles-1925-45.
Now, if you refer to page 20 of that book, you'll find a list of British wartime regulations pertaining to military motorcycles. I would draw your attention to War Circular B.125 "Provisioning for immobilising" (dated 21.7.41). It is said to apply to all makes and models of vehicles. One imagines this Circular might be what you may be seeking for specific guidance about keying and other security. I was unable to find it online through casual Google searches, but the
British National Archives or
Imperial War Museums should be able to provide you with a copy of it or at least refer you to where one may be obtained. I only scanned briefly through that book, so it is well nigh possible that more info on vehicle keying and security may also be described therein, merely awaiting a closer reading of it to discover.
I was able to find various wiring diagrams of wartime motorcycles online, but I believe you may find the
"WD Wiring" webpage of the Dutch-based
BSA W.D. M20 website of particular interest, especially where it reads:
"The wires in this picture are [
sic]
found at a bike breaker still attached to the late war main switch as located below the saddle."I am led to understand that especially as the War wore on, more and more standardization of motorcycles and their equipment was required of all wartime contract bikes, so this BSA practice of an under-saddle switch was perhaps also followed by other manufacturers.
By coincidence, as its ignition switch my old "Desert Sled" '67 Norton N15CS also has a simple keyless battery breaker switch under its sprung saddle left by its previous owner just like this beast:
When in doubt, you might try reaching out to the authors of that book, Chris Orchard and Steve Madden, through their publisher:
Sutton Publishing Ltd.(An imprint of The History Press)
Phoenix Mill
Stroud
Gloucestershire GL5 2BU
United Kingdom
Web:
https://www.thehistorypress.co.uk/about-us/Email:
web@thehistorypress.co.ukYou might also do well to rummage through the couple-few hundred
"Royal Enfield" postings over at the
Historic Military Vehicle Forum. Lotta good stuff there,
like this example.
Hope this helps! I'm really looking forward to seeing what you come up with for that old 350.