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I was able to consult volume XXIb of Le Opere di Giuseppe Tartini , and the sonata in A minor (B a3) is indeed there. Its main base is the Padua manuscript (1888-1 of the Archivio musicale della Cappella Antoniana in Padova) a scan of which is found in IMSLP, but some other sources have been used as...

I guess so, for it is definitely the same collection as far as the 26 numbered sonatas are concerned. Perhaps the catalogue contains an error (A3 for a3; I made the same error in my previous post ...).
BTW I noticed that the manuscript contains earlier sketches of B.a3 on pp. 87 and 89.

The A minor sonata B.A3 can be found in Tartini's manuscript in http://imslp.org/wiki/26_Violin_Sonatas ... iuseppe%29, pp. 94-95, as part of the unnumbered sketches and pieces at the end.