I know you are right here but it is also true that many real New York apartments have crazy things like the hole in the sheetrock.

Who doesn’t want a permanently open serving window between their apartment and the public hallway

I might be wrong but I don't remember the original framing ever showing the sign, so I assumed (perhaps incorrectly) that it was there ready for them to move in shot if they ever wanted it, but that as seen in this image it's not supposed to be on camera

I distinctly remember the "5 card Charlie" sign from the original SDTV broadcast back in the day.

In that case your memory (or perhaps just attention to detail when watching) is better than mine, and I withdraw my previous belief! Thanks for the correction