The International Cessna 170 Association

Frank, does your vent have a "boot" on it's foremost end? If so, a check-ball may have once been installed and subsquently removed. The cotter was to hold the check-ball in position. (And your boot may have been removed with the ball, if it's missing.) The ball is not depicted in the 15 Feb 1963 Revision-IPC for your airplane.

I guess you will have to draw me a picture of the boot George. I don't know what a boot is in reference to the fuel tank vent. I'll bet you are correct that the cotter key/pin is associated with the check valve/ball. Guess I need to know how to obtain the ball & secure it in the goose neck.

No, Frank, the check-balls were subsequently removed per Cessna SNL. The boot retained the ball, and that may be why yours is missing...I suspect someone complied with the SNL which removed the check-ball and discarded the boot as well.