From the screenshots posted
over here it looks to have been filmed in a fairly standard Super 35 format - the 16x9 version has way less dead space at the bottom, is a little bit tighter at the top, with more picture off to the sides. I'd be surprised if it was not deliberately framed this way in the first place, with the true compromise being the 4:3 version we're accustomed to. George's hand and the bowl of fruit on the counter coincide awfully well with the 16x9 frame, even allowing for a bit of overscan on a widescreen TV.


The choice of the 16:9 ratio for the eventual HDTV standard predates "Seinfeld" by years, so it's entirely reasonable to assume they took this into account when filming it.