The thing that's so sad about this is that extension cords are so cheap and so much easier to plug in than this mess.
You don't even need a pre-made extension cord.
You already have the cord. Cost of a realtively inexepensive heavy duty rubber grounded dead-front plug: $3.00 Mating socket $4.00.
Geeeez. I've actually seen people tap into 50-amp 250 volt receptacles in generators like that!!!
At one street fair a year ago I saw a cord stuck into a genny receptacle, with bamboo shish-kabob skewers to keep the wires from slipping out of the socket holes.
What's the going cost of a 50-Amp 250 volt plug these days?