[Shop-talk] Bolt type?

[Jeff Scarbrough]

I'm repairing some outdoor furniture from L.L. Bean.  Two chairs were
put together with carriage bolts, but the third one (bought sometime
later) has a unique type of fastener.  It looks like an elevator bolt,
but the flat head has a hex indentation, so it can be driven with an
Allen wrench.  The underside of the flange is round instead of square,
the same diameter all the way down.

These are much better than carriage bolts for things that need to be
disassembled more than once (and that's these chairs...)

Anyone got any idea what it might be called, or where I can get some?

TIA,
Jeff Scarbrough
Corrosion Acres, Ga.