Yup it is the old English measure of drill system in letters. "P" if I recall off hand. The whole Mauser is built in whitworth. There isn't one metric part on it.

Probably stems from the fact that the machinery used to drill the barrel would not have had any metric bits available in 1898.

Look at a good tap drill chart it is funny as shit. All the "metric" calibers are letter designation sizes. And most of the old time calibers as well. .404, .375. .275

All the screws and threads on a Mauser action are inch measure sizes as well. Some odd ball like the two main action screws which are 1/4" 22tpi.