Why are there two different carburetor openings?
Any advantages/disadvantages to either one?
Can you run any type carburetor on either?
Can you use a carburetor gasket with a 4 hole opening on intake #2?
What is the plugged off boss for on intake #2?

they are both a square bore carb flange. a spredbore carb will need an adaptor to bolt onto it.

IIRC, they 4 hole helps vac signal to carb and adds torque to bottom end. like adding a 4 hole 1" or 2" spacer for more plunum area.
the 4 hole is not divided down the middle and the duel ovel is divided down the middle for a left/right bank split.

I would not use a 4 hole gasket on #2 intake as it can sag/burn/break and get sucked into the cyls.

the plug is a vac port for a vac tee to run brake booster or pcv if needed. just like #1 intake.

I don't recall how to ell B engine from RB engine but think it should be 3/4 inch wider for RB engine?

When Mopar switched from Carter carbs. to the Holleys, they went to the semi open plenums, why I do not know

If you look at the intakes carefully you see that each plane feeds both sides.

R.

Number one I believe is a pre 67 intake and has the smaller carb bolt pattern. It might be a little shorter, and the runners maybe smaller too.

Your correct on Mopar using the Carter AFB up to 1967 and then the Carter AVS for several years, they also used the Holleys with that open intake on some of the low po non magnum motor cars and ended up using them, Holleys, on the 1970/1971 383 Magnums

He just picked that as an example I think , 68/69 383 and 440 intakes have a 4 hole pattern like that. I think there is even a version of the 70 383 intake that is 4 hole also, I think I have one .