It looks suspiciously like a TEMPORARY chain link pin that we used to use to "roll in" a new chain on the old Honda motorcycle OHC engines. You would press out a link pin, attach new chain to old with a similar pin to what is pictured (it was designed with the Allen recess due to some pretty close quarters in the air-cooled mc engine, if I remember), roll in the new chain, REMOVE this temporary pin, place the the new permanent pin and deform/mushroom it with the little press. We looked upon our little Allen bolt as a TOOL, not meant to be used to hold the chain together when fired up. Really have not seen anything like it since the "C" clip was used most often and ours was much smaller than the one pictured and lacked the nice taper. This was ~35 yrs. ago, so I could be wrong.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)