I’ve never seen anything like that on any mechanical fuel pump. Pressure regulators would be located on the outlet side in the line leading to the carburetor. It might be something that was added by a previous owner at some time during the life of your car. I have seen some goofy add-on devices in the backs of old magazines in the past. This might be one of those. Maybe one of the Jetfire owners might know if this is something specific to the Jetfire? Now I’m curious.