Don,
The lifter is VERY hard...it's hardened. It would be a chore to drill a hole in it. Grinding is the perfect operation for a hardened item. I'm not totally sure where this shallow slot is ground, but it sounds like it 'vents' any trapped air to the lifter bore (atmosphere) before it even gets into the lifter.
I have never heard this theory of air getting trapped inside a lifter but I'm just a greasy old Millwright.
Stuart
