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.
UPDATED 1/05/2018 : Over 100 helmets have been added , I concentrated on adding beaded Luftschutz and Polizei shells and they also feature in the breakdown statistic at the top. Many other types have been added as well.
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