Some smooth castings have slots cut into them like this to allow for a pry bar to be used to pop the two halves apart. This looks like a machined cut out on the one half only which would give the impression of a slot cut into the case.
I would guess that on the other side of the crankcase you'll find another similar cut in one half of the case so that the two pry bars are about 180 degrees apart for disassembly purposes.
