Ronn Lucas, who plays the ventriloquist in Harry's flashback, also played a ventriloquist in "The Next Voice You Hear" (4.1) where he ended up with a dummy of Bull.
Christine's flashback takes place at a 1973 beauty pageant. It is revealed that Christine is 19 years old at the time making her birthdate either sometime in 1953 or 1954. This makes Christine 3-4 years younger than Markie Post who was born in 1950.
The judge in Christine's flashback said Christine's measurements were 37-23-35 which were indeed Markie Post's measurements.
Final casting of Stanley Brock (Marty Melman). He played Lieutenant Gerard in S4E11, Mr. Douglas in S2E19, and Vincent Frago in S1E5.
The presence of a live studio audience/laugh track is completely missing during Harry's flashback scene, and there seems to be at most two audience members providing the applause and laughter during Christine's, possibly due to the fact that her scene is a beauty pageant. However, during Roz's flashback (as well as all flashbacks in part 2) it is back to normal.