What does "counter" refer to in "countersink"?

The conjecture on this in the OED says

Etymology: counter- prefix, apparently in a sense akin to 8 b, the hole being the counterpart of that which is to be sunk in it.

My grandfather was a blacksmith (now long dead). My understanding is that a countersink (also called a bob-punch), is used to form a depression in the surface of plates to be joined together when rivetting. Without this, the head of the rivet projects entirely and is subject to damage. If the head is broken off, the rivet will sink into the plate. The indentation counters any tendency to sink. I suspect the term also applies to indentations for horseshoe nails.

Unfortunately my grandfather and my father are deceased and I can't find anything online to support what I'm saying.

I'll leave this here for now and see if I can find anything further.

Google's Ngram viewer returns results for 'countersink' that indicate that it was a new word in the early 19thC. It does not appear in the corpus at all before 1820 and did not take off until about 1860. This indicates that "countersinking" was a process associated with industrialisation.

The Lexico entry for counter gives an uncommon definition which seems to be relevant.

The back part of a shoe or boot, enclosing the heel.

For which it gives the origin as

Mid 19th century abbreviation of counterfort ‘buttress’, from French contrefort.

The mid 19thC origin of the shoemaking term matches quite well with the appearance of "countersink" in the literature and "countersinking" a hole so that the head of a screw or other fastener fits below the surface certainly improves the strength of the join and could be said to "buttress" the fastener. I suspect that "countersink" also derives, perhaps in part, from counterfort.

The etymology seems very murky, but my way to remember the meaning and difference between countersink and counterbore is thus: if you are building a countertop (furniture), then you don't want screw/bolt heads protruding. So you bore a little extra for the screw/bolt head, hence 'counter-bore.' Then if you slope the walls of your extra boring, it resembles a drain aka sink, hence 'counter-sink.' I know that this doesn't line up with the past-tense 'counter-sunk,' but the noun "the sink in your kitchen" and the verb "to sink into quicksand" are obviously common in origin.

From the other couple of languages I've studied, it seems that because english lacks a lot of explicit grammatical features of other indo-european languages and instead relies more on context of usage, the process of noun-ifying verbs and verb-ifying nouns can have ambiguous results.

All in all I think it's simply too murky of a history to try to pluck a thread of causality from unless you're a scholar poring over primary documents, and whatever mnemonic you can come up with is good enough.