Grandpa's old 72 aka The Green Beast

[thedudeabides86]

Hey everybody! I love this forum board and have learned sooooooo much from it over the past few months. I've got my grandpa's old 1972 C20 3/4ton pickup that I've been working on lately and had a question. In this picture, there's a fitting to the left of the coil with a steel line running down to the transmission. Can someone tell me what that fitting is and what it does? I think it's the vacuum modulator, but I've been learning as I go. I read on here once a guy had mentioned that he didn't inherit his truck when his dad or grandpa died, he was "just the caretaker of it." I thought that was really cool and like how it sounds. Thanks for all the help and advice. I really appreciate it.