As I recall, Beretta's xtra grain is a grain enhancement process that involves chemicals and lasers to impart a fake grain pattern into the wood. Supposedly, the process has some depth into the surface of the wood, unlike typical grain enhancement or faux grain painting processes that are just applied to the surface. But I have no idea how far you could sand safely without cutting into the plain wood underneath.
I'd assume (Guessing here... zero actual experience...) that you could lightly scuff the surface with steel wool or use liquid sandpaper, and then apply coats of a grain filling finish such as polyurethane. Then you would very lightly cut this back between coats until the pores were filled, and then apply a final coat. Of course, this would involve masking the checkering, and very probably re-pointing the checkering to remove any finish that bled under the masking tape, or was damaged by sanding. In pictures of stocks with xtra grain, the fake grain runs through the checkering, so I don't know if the process is done after checkering, or if it goes deep enough that standard checkering doesn't cut through. Or perhaps the checkering is impressed. I also have no idea what Beretta used for a finish, or what kind of top-coating would be compatible with it. Sounds like a lot of uncharted waters here. But certainly someone somewhere will refinish or repair one of these, and learn what works and what doesn't.
