Number inside side plate...

[mrerick]

You'll also find this in DWM Lugers, just a little lower and under the area with the exposed part of the axle.

It was used in the manufacturing process to associate one hand fitted side plate to one pistol among the 10,000 in the suffix group. The side plate is precision hand fit.

This was most likely needed for keeping the gun and the side plate associated through batch related processing after the hand fitting was performed.

At some times, the digits inside the side place were the same as the first two digits of the serial number. At other times, they were the first two digits plus one (or the first digit plus one in the case of a three numeric digit serial number).

Some Lugers were made with this interior side plate number missing. This was likely because it wasn't needed to keep the batch together and properly associated through manufacturing steps.

I believe I've seen a DWM or Mauser manufacturing process step flow document somewhere, but can't remember if it was in Mauro and Gerben's book, Sturgess or one of Jon Speed's books. It didn't have details of the internal factory part numbering, but may indicate a step involving a large number of parts being finished in some way...

We discuss this in the FAQ document also.

[Norme]

Internal numbering goes back to Imperial times. Navy side-plates had the first two digits of the serial number stamped internally. The first or second digit was stamped ahead of the receiver lug, but not for all models.
The short answer is that we don't know why they did it, but it probably seemed like a good idea at the time.
Norm