The Monument 361T is a heavy-duty immersion heater spanner used by plumbers, with a cast box-ring head and an octagonal base. To use it, slide the box-ring head over the immersion heater nut and turn it to loosen or tighten. If the heater is very tight, use a hammer to strike the octagonal base on the spanner's handle for extra force to break the seal, as it's designed to withstand heavy impact.