The attached link gives you access to a database of helmet lot numbers which I have started in 2006.
The helmets in this list are all authentic WW2 helmets , I checked each and every one of them and many have been posted on the German Helmet Walhalla forum. All errors of course are my own.
EXAMPLES OF LOT NUMBER TWINS
2 ET helmets with identical lot numbers , one is Luftwaffe and the other a Heer one.
Photos courtesy of RonR
Here’s a prime example of 2 SE Polizei helmets with an identical lot number and are also of the same branch (this does not need to be the case).
Photos courtesy of Mr_S