I've got a 1970ies Sony stereo with NEMA plug and an unswitched outlet. When I push in a plug I can feel that the prongs get locked when fully inserted.
BTW, I was close to using the mexican style cord. I replaced a light switch in a room without a window, so I needed some temporary lighting. The only thing I could find was an imported US table lamp with an adaptor which has been hooked up forever in our living room. When I tried to hook it up I nearly got a fit. The adaptor was as stone aged as the lamp and it wouldn't fit Schuko receptacles! Finally I pulled two (illegal) ungrounded extension cords through the appartment and hooked my lamp up. SO much about being able to stick to codes...
