A sonata for violin and double bass by  Mozart? So it is! This sonata is build by the mouvements No 6, 7, 9 and  12 out of the 12 "Kegelduette" for two bassetthorns. This is quite good  for the concert practise because 4 mouvements of Mozart as a sonata in a  concert are better than to confrontate the audience with all 12 pieces  what can be quite rather long-winded, although it is all together a  rather pleasant music, just Mozart. But in a well measured serving of 4  mouvements it is a real enrichment of the double bass literature which  is not undemanding for the bassist also at all, because many parts of  the bass stay in the violin system, while for the violinist it is more  like a pleasant practise.
The "Kegelduette" are also available in a complete  arrangement for cello und bass and can be heard on the CD "cello bass duette".