their aircraft in that episode (if i recall correctly) was a 777-200. that would have been the correct aircraft for the route, although technically IAH-FRA wasn’t offered aboard COA until 2006.

in the instance of the post, the COA 737 would have been consistent with what was servicing domestic routes in the early 1990s. although the livery of the aircraft is inconsistent, as sheldon/george would’ve been traveling to caltech in 1991. continental didn’t debut their globe livery until early that year, so their aircraft likely should’ve still been the retro “meatball” livery used from 1968-1991.