When I was growing up in the 1960's I managed to get hold of a big Leviton catalog. I remember it specifically included a duplex grounded 277 volt 15 amp receptacle. They also had both grounded and non-grounded plugs for it. I hadn't actually heard of 277 volt utilization at that point, but after playing with some math on it a bit, I figured out it came from 480 volt three phase systems. So that's why it stuck in my mind.
They also had receptacles for 120 volt up to 30 amp, and for 240 volt up to 50 amp. But for 277 volt, the 15 amp model was all there was and it was only in straight blade; there was no locking 277 volt receptacles at all.
So I wondered what would need to use 277 volts. The only conclusion I could come to was portable fluorescent lighting units.
The point there is that if someone needed to have a way to plug to 277 volts with more than 15 amps, they very well could have had no choice but either using a 240 volt receptacle configuration, or hard wire it. I don't know if any other brands might have had something larger in 277 volt, but Leviton certainly didn't in that catalog (about 500 pages).
Even today, with downloaded PDF catalogs from both Hubbell and Leviton, I don't see 277 volt receptacles or plugs above 15 amp, even though the NEMA PDF shows larger ones.
