What would you call this fastener, and where might I buy some more of them? Thanks.
The object pictured is a 1/4" NPT Female Adapter Vacuum Fitting, likely made of aluminum or stainless steel. It is used to connect ISO-KF flanges to 1/4 inch NPT male pipes.
Or so AI image search tells me.
Ai image search is, surprisingly, wrong.
That's a seatpost nut I suspect?
No official name for them afaik, they may well be custom for the post head. Best bet is to find someone with a knackered post and charitable nature.
Knowing where it's from might help?
It looks like half of a countersunk sex (or Chicago) bolt but the two flats to stop it rotating will make it difficult to find off the shelf.
One of those times where you might need to go back to the manufacturer
Could you just replace with a flange nut?
Thanks all for the suggestions - it's part of a wedge for my timetrial bike bars, similar principle to a seatpost nut, long bolt comes in from below and needs to screw into something.
dove1 thks for that link - if that is an M5 size that looks good, but I can't see the dimensions anywhere. But I'll check for other seatpost nuts.