Hello Rusty, welcome to the forum!
Is it possible to either scan in the Luger or take some close digital photos of it? Do you know which proofs it has on it and can tell us from the markings pages on your left? I too have a 42 / 1940 and there were approx 150,000 made.
So under the barrel the serial number is 165 and through the gun there should be the last two digits on many parts.
I think others would be interested in the M's that you see and esp would like to see them scanned in if possible.
Is there a story on how your uncle aquired the pistol? I myself am very interested in that aspect and slowly am putting together a booklet on guns brought back by uncles, dads etc.
If you go to your left, you can get sheets where you can write down all the proofs and markings.
Use this page for markings and telling us what they look like:
http://www.lugerforum.com/lugermarkings.html
and this is the ID sheet area:
http://www.lugerforum.com/IDsheets.html
Ed