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 2HPhttp://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.