"D Major" in this excerpt

I'm going through an old lessons on transposing instruments. The sample is an excerpt in the middle of a piece for Clarinet in Bb, and transposed into concert pitch, we get a key signature of G major. The piece is named "Serenade in D major, op. 11" Is anyone familiar with this? Where does the G major come in?

It's a multi-movement work; this extract is from the minuet, which is in G.

Wikipedia

Steven

Thank you, Steven.
KS