When the Lettermen are singing "Blue Moon" as a duo, the song has three harmonic parts. Likely the song was recorded in a studio and lip synced.
Tony Butala plays "Lou," but on his Letterman jacket, you can clearly see the word Tony.
In the opening narration, Dobie describes a concert by The Lettermen at Peter Pryor Junior College which he attended with his Dad. In scenes of the concert, the first shot of Dobie in the audience shows him sitting next to Maynard, and none of the people around them is Dobie's Dad. Dobie continues his narration, and the audience is shown again, this time with Dobie sitting next to his Dad (and Maynard is nowhere to be seen).