ECN Electrical Forum
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6-50 is probably a more proper choice but the 10-50 is more prevalent. The retention force on those plugs is high enough that they won't be falling out.
If you really want something secure the IEC 309 67 series is good but they tend to be pricy. We started using the 309s in the computer biz when Russell Stoll fell out of favor.

That's one reason I'd have rather used a L6-50. The L6-30's at work (each computer cabinet is fed by L6-30R hanging from conduit+box and UPS's built-in 8 foot cord and L6-30P reaching up to connect) were easier to plug in and unplug than my 14-30 on the dryer at home. So I suspect a L6-50 would be easier than a 6-50. Yeah, those straight-blade connections won't fall out without a lot of force from somewhere.

The 309's do have the advantage of a shroud.

I looked at the twist locks and the Hubbell CS8269/CS8265C says it is 2 pole 3 wire but it looks like it can do 4 wire too using the connector shell.

http://www.hubbellcatalog.com/wiring/catalogpages/section-b.pdf

page B-32 & 33

Have you looked at the 'Meltrics' line of receptacles and caps? Pin & sleeve design, thermoplastic bodies, and some are rated as disconnects and satisfy LOTO requirements.

www.meltric.com is the website.

They sent me a sample setup a while back for 'show & tell' at a CEU meeting. I have no idea of $$$ (edit)

Those do have designated pins ... ground, neutral, and hot in the 125V version, and ground, hot, hot in the 250V version. The 125/250V version on another page does designate the shell as ground.

I have seen those before. I recall seeing their high ampacity version somewhere in person. I would bet they are pricey.

That Hubbell does look like it should work for you without going to Japan.

I guess I should not hold any real preference for NEMA plug configurations. Of all the designs they have for many voltage, system, and amperage configurations, there's only one pairing that is compatible between 120/240 and 240 volt systems, and that's 6-15 vs. 14-15 (e.g. a 6-15P will plug into a 14-15R). Maybe I'll write more some day on how a smart system could have been designed.