well the Jefferies tube in The Ultimate Computer is labeled
Circuit Bay 121
CIRCUIT BAY G-121
Yes, they repurposed the signage that had been used in two different places during the first season.
The other place it appears is on the wall of the corridor opposite the door of Engineering when Kirk runs in to fight Khan at the end of "Space Seed." (I really do need to grab a shot and get it posted.) But you can see it at Trekcore:
For our Phase II
sets, the different signage looks something like these samples:
I never heard where the Jefferies tube was supposed to be running until Franz Joseph made them climb up to the nacelles. I think I read somewhere that Jefferies had "intended" it to run up the spine of the connecting dorsal. I don't remember where I saw that or if I'm even remembering it correctly.