M1917 Heat Stamp Batch Numbers? ZF41

[M16Man]

I was wondering if helmets with the same heat stamp numbers were made at the same time. For example, my helmet that I just bought has the heat stamp ZF41. I know that the letters in the stamp represent the manufacturers, so ZF represents whatever manufacturer. Would it be safe to assume that the numbers following the manufacturer letters correlate to a batch number? I found another helmet for sale with the same ZF41 heat stamp with a liner dated to 1917. I've also found several other helmets with the ZF41 stamp. Would it be likely that these were made in the same batch around the same time? I've also found several ZF15 stamped helmets for sale. The manufacturer stamp is the same, so I'm thinking the different numbers (15) has to be a different batch/lot or something like that. Has anybody else noticed this or have any information about these heat stamp nunbers? Thank you