I have a 1940 Enfield.
One thing you should be able to get is the contract number, which is what the War department would have used to order a certain number of each machine. They are dated to a month and year. So, my Enfield was under a contract placed in November 1940.
Engine/frame numbers would be associated with these contract numbers.
Contract numbers usually get attached to bike, often painted on the tank.
Post war, you are still reliant on still having the original number and being able to trace it back through vehicle records. I know mine was registered in West Wales in 1946, as I have the original plate.
I found the guys on the WD forum really useful:http://www.wdbsa.nl/
They are not just BSA focussed.
I have a couple of useful books with more pictures of military bikes:
Motorcycles at War - Gavin Birch: http://www.amazon.co.uk/Motorcycles-War ... 805&sr=8-3
British Forces Motorcycles 1925-1945 chris orchard/steve madden :http://www.amazon.co.uk/British-Forces- ... 886&sr=1-3
The second book has a list of contracts vs engine/frame numbers in the back.
send me yours and I'll see if it's listed
John "Josh" Nashwebmaster@arielownersmcc.co.uk
2009 royal Enfield,1978 t140 bonneville, '67 CJ750, 196-ish Ural M62 outfit, 1960 k750, ''51 kH500, ''49 soon to be Ariel bobber, 47 VH twinport, '44 Ariel WNG, '43 Ariel WNG, '41 Ariel WNG and piles of rusty scrap ....