A sonata for violin and double bass by Mozart? So  it is! This sonata is build by the mouvements No 10, 5, 11 and 8 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".