This is a cheeky arrangement of Ludwig van Beethoven's String Quartet in F major, Op. 18., No. 1, reimagined as "Happy Birthday".
The quartet was written between 1798 and 1800, and published in 1801. While it is the first quartet in Opus 18, it is actually the second quartet that Beethoven composed.
Form-wise, this is pretty much the exposition played twice, but the second time adapted to F major (as it is in the recapitulation in the real thing).
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