On another Forum I helped a member to date an ammunition box by using the Lot Number. I thought I'd post this here to possibly help you. Or maybe not.
Each Ordnance Plant was assigned a series of lot numbers during WWII. For example, Lake City was assigned numbers 12000 to 13999. But - this is important - each Plant used the same numbers for different types of ammunition. For example, at LC, Carbine Ball would start at #12000 but Carbine Tracer would also start at #12000. And, Carbine Dummy at #12000, and so on.
The other calibers that Lake City produced would also start at 12000. For example, Cal .30 Ball M2 (30-06) Lot #12000. So, you can have a lot of ammo cartons and boxes from Lake City with 12000s lot numbers.
The numbers didn't lapse after a certain period of time. For example, Lake City did not manufacture Match ammunition until 1957, but the first lot number of the new T291 Match was - you guessed it, 12000.
The bigger Ordnance Plants may have been assigned additional blocks of numbers. For example, LC was also assigned 35351 to 35999 and 42000 to 42999. Also, if a Plant closed and had unused numbers they could be assigned to another Plant. Such as the above mentioned LC 35351 to 35999, which were originally Denver numbers.
In 1970 the lot numbering system was changed so all of the old numbers ended wherever they were. The system changed again in 1979 and has stayed pretty much the same to date.
This doesn't cover any pre-WWII numbers, experimentals, special lots, etc. I don't want to go there, so don't even bring it up.
This may sound confusing but it's actually pretty simple once you get used to it. It helps to have a military mind set also.
Now, I'm sure there are some members out there who know more about lot numbers than I do and will point out my errors. But, that's how I learn. So correct me where I'm wrong.