I've seen two phase 120/208V used in residential houses. I know its rare but it does happen.
By using a transformer you can convert two phase 120/208V into full three phase 208Y.
I also know of a power utility where all the transformers can be used at either 7.2KV or 14KV due to a planed voltage conversion.
Then there was the one company that wanted 6 phase 120v/63v/208Y/240v
It used three 120/240V services, but each one was coming from a different phase.
Also don't forget that due to all the surplus it's cheaper to run phase converters than to convert equipment to single phase. so you could have 208Y three phase in a home workshop.
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Back to the topic at hand.
The guy has three wires, Hot, Hot, Ground
This means that
1. You have to remove the bonding screw per UL
2. You don't have a neutral and thus can't use the neutral bar.
3. You need to use a ground bar that is screwed to the back of the panel.
This leaves the neutral bar unused for future use and to prevent someone else from getting confused.