Seems evident from the screen grabs that the original production was shot both 4:3 and 16:9 safe; however, not in the way that ABC/CBS and some others programs are...in that they simply "center cut" the 4:3. It's obvious that Seinfeld was shot so that the 4:3 was the primary picture, with enough over shoot to allow a 16:9 transfer to be made. Some of the very old shows...such as "Hogan's Heros" has been running on HDNet, and it's @ 14:9...with a little black on the outer edges. I don't think they had any intention of shooting for 16:9, but left enough safe area that some kind widescreen format could be produced.
If we could see some of the first Frazier's that NBC ran in HD, you'd see that the director of photography had never intended for those first shows to be shown in 16:9, as the "headroom" was a constant, uncomfortable issue. Beginning later that season, or maybe the next, things straightened out and the HD masters were properly formatted.