We just had a topic on this. Start here: http://www.trekbbs.com/showthread.php?p=8690503
and read down.
It was pointed out in the other topic that the screencaps for "Beyond the Farthest Star" reveal the door to be a turbolift.
Closed Caption wrote:
On this page
of screencaps from Beyond the Farthest Star
, there is this
shot, which gives about the best look there is inside the second door that I recall. As one of Filmation's infamous errors, the shot evidently lacks the door elements themselves, thus revealing what's back there.
Earlier on the same page of thumbnails, you can see McCoy
exiting the main turbolift
A comparison of the backgrounds, in particular the colors and noting the presence of the silver stripe almost shoulder high, convinces me, at least, that the intent was for there to be a turbolift at the second exit, as established in the behind the scenes material that's out there.
I still prefer to think of it as Franz Jospeh depicted - access to the bathroom and stairs to deck 2.