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This is an old wire nut that we use to come across every now and then. We heard that it was used in places where there was heavy vibration.

Someone said that the wires were tightened under the screw to prevent the wires from coming apart. I don't know how true it is, but they look like it would work well.

Harold

[Linked Image from electrical-photos.com]

[Linked Image from electrical-photos.com]

[Linked Image from electrical-photos.com]

[Linked Image from electrical-photos.com]

[Linked Image from electrical-photos.com]