An IBM Model F mechanical buckling-spring keyboard, as supplied with the original IBM Personal Computer AT, or PC/AT.
https://files.mastodon.social/media_attachments/files/111/425/500/106/909/287/original/bf7a3081e48fd067.jpg
@Bemused @admin @babe Dunno if this counts, as it's not a Model M - but my Model F was built in 1985. Still fully-functional, using an AT-to-PS/2 adapter into a PS/2-to-USB adapter!
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