The "later" (also called "final") version of the 2nd Mass in G (D. 167), was discovered in the 1980s at Klosterneuberg, and includes trumpet and tympani parts that were not in the original (composed in 1815), not published until 1845, posthumously).
These trumpet and tympani parts were susequently published by Carus-Verlag and are presumably only available for a price (purchase or hire), since they have appeared in print only in the last 30 years. Indeed, if they were public domain, IMSLP would almost certainly have them (and they don't).
The "later" version of the score apparently also has some minor musical differences from the original published version, and of course it, too, is available from Carus.
Ferdinand Schubert (Franz's brother) wrote woodwind, brass, and tympani parts for the original, because of the popularity of the work. I'm not sure where (or even whether) these are (freely?) available.
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