That is a mess.
My guess is someone always left a reed on the piece and acid saliva tore in to the material that is already a silly material to use on a mouthpiece. A lot of material will have to be removed to get a flat table again but its likely fixable. If this is a piece you own it can be saved. If you are thinking of buying it...I would pass unless the price was very low.
Man, that thing looks like it was buried for 20 years. See if you can just scrape any of it off with your fingernail, like on the baffle. Be careful because the pewter is soft. It looks like at least some of the black material is a build-up on top of the original material. The safest thing would be to find the rust remover that would clean it without any sanding so you can make an informed decision on a re-facing. In the USA we have 'CLR' or 'Lime-Away'. You probably have something similar where cleaning supplies are sold, or you could try white vinegar.
If it is a build-up which can be removed without sanding, you will probably be able to use the mouthpiece even if the table has some pitting. If it has to be re-faced, it might not be possible to keep the same tip opening - it could become a 'D7' for example.
I saw that mouthpiece for sale online...yikes. Wasn't it awfully cheap for a D9, and didn't the seller say something like it was "untouched" or "great shape" or something like that? Double yikes. It doesn't look like buildup to me, it looks like the Silverite has been eaten away. Like a reed soaked in Coca-Cola had been left on it and then buried in a termite mound for a year. MIGHT be salvageable, but the table looks destroyed.
The “buildup” is likely corrosion products. When metal corrodes, it forms a less dense structure that swells in volume. When you clean the corrosion away, you’ll find pitting below it. From the appearance of this piece, I predict some pretty deep pitting on the places that count. I would not count on having enough metal remaining in the critical places to restore it to original specs.
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