God only knows, but maybe out of an early '70s Honda Four (had a couple of those). I started with about 12 of those first jets (probably three sets of four), and a gazillion of the 5mm threaded ones (all different sizes). I suspect these "#2" jets were used in about everything.
I wrecked a few before figuring out how to use a die holder as a vice: put in a little piece of metal to keep the jet from screwing through the die whist drilling on it. (Hope that's clear enough to picture.)
Gonna try to screw it all back out tonight to swap the 5mm jet out. It went in clean enough, so I don't think it'll be a wreck.
That motorcycle carbs link has about everything except these secondary main jets. (That's just about everything I really need right now.)