What's this wire doing?

[bitbucket]

Attached photos show this phone in a working state. There are 3 incoming wires. One goes to R, one to Y, and one to the uppermost terminal on the coin relay mechanism

I'm the one who wired this but I had to remove it from the wall and now I can't recall what that third wire is for or what I'd put at the other end of it. With just R and Y attached to the incoming line, there's no dialtone.

The upper housing tests good on a known working phone (which seems to be wired mostly the same except missing coin mechanism) A test-set works when attached to R and Y. 

What's that wire doing?

Edit:  Did I really go through the trouble of energizing the coin mechanism where I could have just wired it out of the picture?

[poplar1]

If this is a 236G, and the wiring is factory, then the incoming line connects as follows:

Tip (Green) to R
Ring (Red) to Y

Yellow wire (ground) connects to G on coin relay, but only needed if you are connecting the phone to a payphone line simulator, or a prepay coin trunk. So you don't need to connect the yellow at the wall.

Here is a 236G diagram:
https://beatriceco.com/bti/porticus/bell/images/236g02.jpg