An archaic or obsolete variant form of countersunk, sometimes used in older woodworking or construction texts.
From counter- + sunken (an alternate past participle form of sink, now archaic). This represents an older English construction where -en was regularly added to past participles; modern English standardized to countersunk.
You'll spot countersunken in old English carpentry manuals and historical building records—it's a ghost word of English grammar that shows how we've simplified past participles over time!
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