This is a wire connector, with a base of some alloy, shaped like a square block with three tubular holes and three set screws, suitable for use in connecting copper and aluminum wire, with the clamping accomplished with three set screws.
Also it is pre-greased with silicon anti_oxidant to control aluminum oxidation.
In a residential size, you would insert maybe two aluminum wires and a copper wire, each wire going into one of three tubes bored into the base, and then secure each wire by the set screw for each tube.
(The manufacturer properly advises to abrade the aluminum wire
before insertion)
The attachment is like using a receptical with screw clamps.
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