These sockets have a porcelaine body and 2 springs forming a semi-circle that grip the pins of the plug. If they get loose, just bend them together again. These devices were pretty expensive, but designed to last forever. The worst thing I've ever seen with them were broken covers, due to severe mechanical force.
I had to do the bend trick with one of the 8 I installed, but the other ones look like new after I cleaned the covers. These old devices contain hardly any parts that can fail, so I think they can be trusted if they pass visual inspection. same thing with toggle switches from that era. As long as spring and axle are in good condition there's nothing wrong with using them.
Newer devices with backstab contacts are far less trustworthy. They basically consist of plastic, and everything is covered up, just leaving small holes for the plug pins.
