Dan, mineral spirits will attack most finishes including the original urushi lacquer on the stock. The urushi is very durable though, and you'd have to really slosh it on to reduce the finish durability. The problem with using most solvents, including paint remover, is that it does not distinguish between the two applied finishes. You'll remove everything.
Ok, please do not use any abrasives, such as sandpaper and steel wool either. Abrasives show no prejudice and attack both coats. Neither coat is over about .003 in. thick anyway, so when do you stop when you try a stunt like that?
We've still got one other choice. It was popular after WW2 to add a coat of shellac to souvenir rifles. And I hope yours is shellac. Please pick up a small container of "completely denatured alcohol" at the paint store, hardware, H/D, or Lowe's. Using a soft cotton cloth soaked in denatured alcohol, pick a place on the metal where the new finish is and rub like hell. Then add some more alcohol to the rag and rub some more. Do you see a discoloration on the rag or removal of finish on surface being rubbed? If so, you're lucky and also have a project. Denatured alcohol will not attack the urushi finish underneath. Do not use rubbing alcohol - it's too harsh and will soften the urushi. I spent about 2 months spare time removing the shellac finish added to a rifle I used to have. But when through, you couldn’t tell the shellac was ever there!
Well, if the denatured alcohol does not work, we're out of chemicals, but I'd be willing to try one more thing. The added finish does not appear to be sticking well to the old finish underneath. That hints at improper surface prep or, if lucky, an oily surface before the new coat was applied. I've never done this before, but I'd try heating the surface with a hair dryer. Set the gun on low first and apply heat slowly on perhaps the underside of the buttstock. Then scrub with a soft bristle brush. Repeat. If you're brave and see that you're not affecting the finish underneath, turn the gun up to a high setting and repeat the scrubbing process.
Good luck on all this!