English Tech/Engineering Mechanics / Mech Engineering gas turbine

in a drawing, sheet used as a legend, where all symbols are defined. This one is a rhombus containing a double number sign and next to it there is the explnatino "instrument insert tap", unfortunately it follows under the category "miscellaneous", so it could be anything, other items that precedes and follows it are: filter, pump, silencer, fan (and the corresponding symbols)

question: what is an instrument insert tap and is there a synonym which is more widely used for this symbol?

thanks!