D and S plugs, awful things!
I came across them at the shipyard in Barrow. The whole yard was wired up with the damned things.
They were hard to insert and remove, needed lots of force.
Most of them had no cable clamp, just a silly rubber washer to hold the cable.
Certain places in the yard had DC mains for some odd reason.
That did not do the 2kVA transformer much good when I plugged it into an extension board which unbeknown to me had 220v dc on it.
Wow did the burnt varnish stink!
JimB