Mystery Catburetor

Total newbie here. I would like to find a fitting for the port in picture 4. I don’t know what to look for (size, thread, etc.). I have searched the number shown in pictures 3 and 4, but no luck. I think the number is 17062757, but that doesn’t seem to exist.

This was on a 1986 GMC 350, but the truck has been pieced together, so I don’t know its true origin.

Additionally, in the interest of education, I would also like to know what the chart to the right of the port means.