Help - what is this bolt

Big Block 1964 727 - So we went through the whole learn about the park cable/spring etc because it looked like that housing was leaking. It was, but we still had a pretty good drip. After lots of wiping and driving and looking it appeared to be coming out of the bolt with the pen pointed at it in the picture. It is just ABOVE the park lock housing, faces rear, and has a 9/16 hex head. It's at about 1:00 in the picture. I'd like to know the function in case I need to seal it or replace what looks to be a sealing washer.

Trans Bolt.JPG