So, here’s your simple solution. Buy one with an artist package upgrade. Boom, done! It’s made for the more OCD about that kind of thing among us. It comes with a shell signature inlay.
Historically, the PRS headstock signature was first a decal with painted headstock; then a gold sticker with a painted headstock, and the available artist package included the headstock overlay with shell. When PRS moved to the standard headstock overlay that matches the fretboard with an inlaid signature a few years ago, the inlay was the corian or whatever plastic it is, and the artist upgrade was still the shell.
Dealers can order Wood Library as a ten top or with an artist package. Evidently the one you looked at was not an artist package.
Finally, it’s more expensive to inlay the shell, and most people don’t give a damn what the freaking signature inlay is made from; I certainly don’t. And I have PS models that have all kinds of fancy BS all over them, so it’s not like I’m a cheapskate. There’s absolutely nothing wrong with the signature being inlaid with the corian (or whatever it is), and charging more if you’re into all that shell inlay.
PRS no longer offers mammoth ivory inlays. I have them on one of my PS guitars, and to me, it looks like Corian. No freaking difference except it’s yellower. Except for the difference in the customer’s head, which is of very little practical value. I’d have been just as happy with Corian.