Image edited for size, color, brightness, and contrast from a garish photo at the
whatisgthisthing subreddit. The discussion thread is reasonably focused and includes an explanation of termionology:
"They’re called Dutchmen, they’re shaped like footballs to cover long knots or splits.... That’s not a Dutchman. Dutchmen are also called bow ties because that’s what they are shaped like. This is a biscuit.... No. It’s a Dutchman. A Dutchman can be any shape and is used to hide blemishes. A bow tie is a Dutchman key and is used either for decoration or to stop cracking. This may be the same shape as a biscuit but it’s not a biscuit because of how it’s used. Biscuits are for joining wood... Can confirm, I work maintenance at a fulfillment center and if a conveyor belt suffers damage one of the options is to cut out the damaged section across its entire width and then lace in a length of new belt to fill the gap. We commonly do 8 ft Dutchmans to allow the entirely of the patch to be inside the pulleys of the main drive and still have both lacing visible and accessible, should the lacing fail then the Dutchman isn't all wadded up in the drive..."
And as to why one would cut knots our of plywood:
"Knots in wood are much more dense than the stringy, normal wood. When they make plywood, they layer thin strips (plys) of wood together and glue them to one another like a wood-and-glue sandwich. The problem arises when there is a dense, brittle knot on either of the two exposed plys on the plywood sheet. Shaving a slice of a dense knot gives you a super brittle portion that often ends up crumbling out in crumbs.
Think of it like having a sheet of paper with a small section of equally as thin glass embedded into the paper. You can bend the paper portion, cut the paper portion easily with scissors, but the little glass portion has different properties. It's more dense, but you can't bend it or stress it or else it will shatter."
Hat tips to the commenters. More at the link. I know this is TMI, but I'm desperately trying to keep my mind off that clusterfuck of the U.S./Israel/Iran war ruining the world economy.
Addendum: As I continued on this topic, I found an entire Wikipedia page on
Dutchmen. Evidently the term is used regarding replacement/repair material in a wide array of otherwise unrelated professions: woodworking, masonry, shipbuilding, railroading, theater, boilermaking. The etymology and connection to the Netherlands remains unexplained in what I've read (maybe it's an allusion to putting a finger in a leaking dike).