The image shows the engraving on a Linhof 2x3 finder. "2x3" is the nominal term for 2.25x3.25" sheet film, but the actual size of the format can vary with the filmholder. 6.5x9cm is the European sheet film format. The groundglass back can take both sizes of sheet film holder (both holders have the same external dimensions), but the size of the film is different. So this holder should be able to take masks for 2x3", 6.5x9cm, and all the smaller rollfilm formats that Linhof made backs for (6x9, 56x72mm, 6x6, and possibly 645). If you want to use sheet film, use whatever you can get most easily. The masks have to be the right type to fit the viewfinder physically (either the knurled metal type or the type with the rubber ring, depending on which finder it is), and they need to be masks designed for a 23 finder, and not a 4x5" or 5x7" finder.
I don't know the model numbers.
Bob Salomon used to have that information, and might still give it out on the forum at lfphoto.info, but he's retired, and might have better things to do than looking up serial and model numbers for Linhof users. If you're in Germany, I'd contact the factory. They should also be able to tell you whether they have masks for the last version of the finder in stock, or maybe they even have a finder on the shelf, but brace yourself for the price of a new one.