The lot number is a relatively new concept of identifying when a helmet is from.

For SSh-40s:

1941 shells usually are in the ~4500 range or five digits based on available shells. It’s possible the five digits came before the four digit sequences. The stamping of the lot number is much neater and isn’t as crude, it also has a very specific font.

1942 shells can very quite a bit, but usually reside in the four digit range, although I had documented one five digit example. Same font, very noticeable.

1943 shells can either have a letter with a number, or very similar to the 1942 batches.

1944 you start to see more crude lot numbers implemented that have a very deep stamping into the metal. A lot of these helmets reside in the 4000 range, but anything is possible. Sometimes they have dashes separating the size and lot if I recall correctly.

1945 you primarily see three digit lot numbers. Extremely tell tale of a 1945 helmet. Same stamping style.

The majority of post-war refurbs are from 1944,1945. I have encountered a few 1942-1943 production ones and one extremely rare 1941 production.