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?