Photo shows 30A 250V (upside down) stamped in the socket, with two flat blades and one round, so it's a 6-30 as
@Sophias_dad states.
Note that when you plug in the NEMA adapter for the UMC, it's going to be upside down; the power cord will go UP. This assumes you took the photo right-side up or that the photo wasn't rotated.