He wrote this very close to the time he wrote Symphony No. 24. I have no idea what “dB” means, but it appears to be a correction in the K. number of his works.
This is the first of two symphonies Mozart composed in G minor, sometimes referred to as the “little G minor symphony”. (The other is the Symphony No. 40), also in G minor ans sometimes called the great “G minor symphony”. More important – these two symphonies, both in G minor, are the only ones in minor keys out of 41.
The second movement is in Eb major, and this is unusual because normally something in G minor moves to Bb major. Eb major to Gm, in chords, is a morph and thus more subtle and became a favorite choice of Beethoven.