I second the notion that the excavation were made by a center bit. I have seen
this same pattern in the bottoms of excavations of the recesses for the firing
lock mechanism in 18th and 19th Century muskets and shotguns. Often the marks a
two or more diameters of centre bits are visible in the bottoms of these
“P.S. If you do not receive this, of course it must have been miscarried;
therefore I beg you to write and let me know.” - Sir Boyle Roche, M.P.
I (very) strongly suspect (given the period, and diameter of the holes) that
those are the traces
of a centre bit. Cheap to make using hand methods, and very effective.