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
recesses.

Cheers,
John

“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.

BugBear