Questions?
Provide framed rough wall opening 5-15/16" wide x 4" high (150 x 102mm). Minimum recessed depth to finish face of wall is 4" (102mm). Insert unit into rough wall opening and secure with two sheet-metal screws (not furnished). Backing must comply with local building codes
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and asks like a clamp in that when you loose attach it to the back of the soap dish and slide the entire assembly through the preparation in the wall and tighten the screws, the clamp is drawn down onto the back side of the wall "clamping" the soap dish onto the wall. This clamp is specifically intended to be installed after plaster as sounds like the correct item for you application.