I see a ad in my new "Shotgun News" for WWII P-38's with ""matched" "mismatched" or "force matched" ser:#'s.

OK, matched & mismatched I have no problem understanding. But what is a "force matched" number? Just a part from another gun with the# changed? If so, why don't they just say some "replaced and renumbered" parts?

'Force matched' is just what you guessed.

The (marketing) thinking is that if the renumbering was done by some unknown Military Arsenal somewhere, it's supposed to be worth more than if done by a money hungry surplus arms dealer.
You have to guess which.

Force matching is common on many milsurps, especially Mosins. They don't use "replaced and renumbered" in ads as it takes longer to type.

You also get subsets within force matching. For example there is "Finn matched" with Mosins which refers to the particular parts that have been remarked while others are either scrubbed or left with the original. Then you get those who like to distinguish between renumbering by grinding and restamping versus a line out and electropencil job. Quite a lot of snobbery out there as you can see.