Look at the 6-20R/6-20P 230V receptacle/plug image:
a) why are they different? (Just curious; looking at another person's project)
b) it seems that the horizontal part of the receptacle opening is permissive and not restrictive. For what plug is it permissive, other than the 6-20P?
Beat me to it. Yup. Have seen these but somewhat rare. Usually at mountaintop radio tower sites where driving back down the hill on a poorly maintained USFS road, to go get the right plug, would be a PITA. No reason to block the lower amperage plug, but many pieces of gear need the higher amperage and the circuits are built for those.
Yes, just like the 5-20 receptacles that you may have around your house. The 15/20A I think are the only ones that have this compatibility. Everything else is one-to-one.