Kern hand-held single prismatic optical square with ribbed handle and grey plastic cap. There is a recessed hook on the base of the handle to allow a plumb line to be attached to the device.
For description of its use click here.
This optical square was passed down to the donor from her grandfather Alec Gilbertson FICE FIMechE who was a chartered civil and mechanical engineer with world-wide experience. To see a copy of his extensive CV click here.