It is likely a 6-20, but he said it was odd. Most people have seen a 6-20.
You have never seen a crow's foot 20 A used for a window unit? I have replaced quite a few. Apparently there was a phase in the 60's and 70's where some A/Cs had 125/250 V ungrounded plugs. I don't know why, but I've seen enough 10-20s installed to know that it happened quite often.
I always end up replacing them with either a 5-20 or 6-20
Do you just use the neutral for ground when you replace the outlets?