Usually wilbur with these sixties Victa mowers the 4 stroke base is made as a 2 stroke base then the centre section is cut out and the 4 stroke mounting holes that are already cast into the base but not fully drilled through are drilled through for the 4 stroke motor.
I don't own a 4 star or the 7A 4 stroke but from memory when seeing an original 4 star it had the bigger centre hole, that makes me think people are drilling their 2 stroke base for the 4 stroke motors but are not removing the centre section although the 4 stroke / 4 star motors have a spacer plate to lift the engine so it looks like the centre section doesn't need removing except I'm assuming the oil drain plug is under the motor meaning if you don't remove the centre section of the base, oil changes will be more difficult.
With the reinforced base that I can see in your images I'd say your base is 1961 .
Logically I would say your base is a two stroke base that has been drilled for a 4 stroke.
I have an image of a Sheerline base that shows the four stroke holes are not drilled through.
