electure, I’m missing your question. The two-blade devices I referred to are 2-20.
The three-bladed device are NEMA 10-20 and up. These are officially {by the NEMA WD6-1997 standard} "non-grounding" devices.
Sven’s second-to-the-last picture looks like a NEMA 6-20 and the last looks like a 6-15. These are officially "grounding" devices.
