A Beethoven quartet puzzle: in the last movement of op.18 #1, coda, a catchy alien tune appears out of nowhere -- maybe from a popular opera? Mozart? -- and first appears in measure 346 in the cello, then in measure 350 by the viola, and finally in measure 354 by the first violin and second violin in parallel octaves. It is far from hidden, and has nothing whatsoever to do with the basic material that the movement is built on.

Nobody ever mentions this melodic reference by Beethoven, which is cute, clever, and very obvious. Do you have any idea where the tune comes from, and why Beethoven might have used it? Beethoven certainly makes a point of it, yet I can find no mention of this quote, where it comes from, and why Beethoven used it.

Thanks for any clues you could give me.