At www.nooutage.com/pkg003.htm note their comments about substitution of a NEMA 14-20R for the 6-20R. In part it says,

“NOTE: Depending on availability, we may substitute DeVilbiss #GT5250. This unit is identical to the above except it has slightly higher ratings of 5250/6500 watts and includes a NEMA L1420R twist lock receptacle in place of the NEMA 6-20R. The twist lock receptacle allows use of a standard cord set without the pigtail…”