What's this spigot on my mechanical fuel pump?

[Mikesbaron]

I don't know much about my 1979 other than that it has some sort of Holly Carb and a lot of piecing together. The fuel pump had this small spigot plugged off. What is it for?

 

[CarMichael Angelo]

When you uncap it, it allows fuel to flow out, hopefully splashing back enough to work as a solvent to wash off all of the greasy crud off that has accumulated on it.