Intake manifold coolant passages open/blocked???

Ok, I know this is deja vu but I've been researching previous posts on this issue and can't find a definitive answer....can someone give me the 10 cent version on why some Chevy SB years have the rear coolant passages blocked and some don't??? Will it affect the cooling properties on the directional flow of the coolant??? Will it affect the operation of the thermostat???

Like for instance...a TBI intake will have the rear passages open and the heater core line feed port between the passages. In this case I'm assuming the passages will need to be open and not restricted???

Need some confusion clarified......Thx in advance!!!