[Submariner616]

On my 1960 C10 283 with 2 speed powerglide that I am restoring, there is a hole in the firewall next to the master cylinder with a light blue plastic "pipe" through it. Inner diameter is 1 and 13/16th inches, outer diameter is 2 inches. Please see photos.

Does anyone know what this is? Is there a plug that is supposed to go in there?