These works are scored for string orchestra.  They are entitled
"divertimento" on the autographs.  Some think that Mozart wrote these
in short hand as sinfonias to be fully scored later on his trip in Italy.

During that time there was still a bit of juggling musical categories
around.  Some of Haydn's piano sonatas were also called Divertimento for
Piano; and a few of his symphonies were thrown into that category also.

Some even wonder if they were meant just to be played by String Quartet or
a String Orchestra.  But K136, 137, 138 are all marked as Divertimento by
Kochel and the title of Salzburg Sinfonias is just something confusing,
done by record companies.

Kim