Ok, your carb is not original...it is from a 1975 truck, which is not necessarily a problem.
As already mentioned the screw hole is for the anti-dieseling solenoid...solved, no problem.
As for the fast idling...before you adjust the carb you need to get a timing light and correctly check and set the timing on your engine....always before you adjust the carb. always.
Then, start by checking the idle screw on the drivers side of the carb to make sure it is not dialed in too far....is it touching always? check.
Then, if not touching you will need to check the fast (choke) idle screw on the passenger side of the carb...partially hidden way down low...is it all the way in or where is it at?
Also, I'm wondering what port on the carb your vacuum advance to the distributor is tied into ? ....hopefully you don't have it connected to full vacuum.
Usually the vacuum advance is connected to that plugged port right near the anti-dieselling solenoid tap that you have identified.
Check this out and let us know what port your vacuum advance is tied to...because if it is always advanced it will increase your idle speed.
...ok, lets start there.
All good
Coley
