What is this and why would it be there?

They gotta machine the lifter bores and if there is any part of the block sticking out in line with the lifter bore axis from the deck on down, it is going to get machined. Right? I can't see why you would add these slots there on an engine that didn't have them? The lifter is .904 diameter as is the bore. What do you need a cylindrical relief for that far above the lifter hole? I The tie bars aren't that shape and the slots are deeper up by the deck. I guess the roller lifters are longer, but then that slot would have to be offset from the center of the lifter bore? Guess I'll have to look at the bare block in the garage and see if my feeble mind can grasp it.