[John Sabato]

Mauser sideplate interior numbers "typically" are a one digit increment over the two numbers that make up the first two digits of the serial number.

For example, the number of the pistol you mention is

8229f

I would expect to find the last two digits "29" on the outside of the sideplate, and the number I would expect to find on the inside of the sideplate would be "83" as that is one number increment over the first two digits of the serial number.

I have never seen an explanation as to WHY the sideplates were numbered in this manner, but have seen dozens, if not hundreds of examples of Mauser manufactured sideplates that were numbered this way.

I would welcome theories and speculation on the "WHY"...

I would further speculate, that in your case, your sideplate originally came from a Mauser pistol whose serial number was only three digits..."129".