I see your problem
When I look at a 6-50 they just list that at 3HP. The same site has a 6-30 that says 2HP
http://www.stayonline.com/detail.aspx?ID=4740
They must want the plug to be rated for LRA or something.

All that said I have a 5HP pressure washer that came with a factory installed 6-30. It is a GFCI plug that is "new standard" so it has to be reset after the power is applied so the plug should never be able to be inserted against LRA.

Greg Fretwell