What size front sight mates with a rear sight stamped "M"?

Not trying to discourage your quest, but there is evidently not much information about the SIG P250. An armorer course for the pistol was advertised by the SIG Academy at one time and if I recall all the events at the time, I registered for it and then was notified that the course had been canceled. I suspect that although it’s a great pistol in my opinion, it was so unsuccessful in the marketplace that SIG decided to give up supporting it very quickly. I don’t know if an armorer manual was even developed for the gun, but if it was and if you can find one, it might have the information you’re seeking.

Hopefully you've seen the response to your post in the Lounge. (FWIW, duplicates are usually frowned upon in the forum.)