Rylands Bros. Precision Plate No.12

Precision plate - 'A precise flat plate usually used as a benchmarking tool. Its flatness will be changed when the environmental temperature deviates.' Keeping the mirror flat is important. Perhaps design (with small split running through the middle) helps with this?

Precision plate includes a circular mirror that does not warp objects, firmly attached to tan wooden slabs which are attached to a darker wooden box with lid. Box comes with a key.