That is a PVS valve used to control the vacuum from your carb EGR port to the EGR valve. The valve's intended function was to aid in warm-up driveability. Lol...some never made it past the trash can when the new owner and car got home. They can be identified by color code or engineering #. The most common valve for a '73 351C Mustang is D3TZ-9D473-A which could be blue or have a plain top. ID numbers changed several times through the '80s. You can use valves marked D3TE-AA, A1A, A2A, D5DE-B1A, D5ZE-A2A, D7ZE-A1A, A2A, E1SE-A1A, A2A, AND 77TF-A1A. I have found these on E-Bay by searching for "D3TZ-9D473-A".