RAF surveying tape measure

By Chesterman of Sheffield.  50' Surveyors' tape measure.  Hard leather cased with solid brass fittings and a very neat fold-out hinged winding knob.  Marked as 'Chesterman's Treble Clef',  the actual measure is marked in feet/inches one side and metric on the reverse.  The case is in very good condition with a lovely patina.  We haven't pulled the tape out all the way but what we have seen is in good condition bar the end which has separated slightly from the leather backing and the end loop lacks a rivet.  There is some movement in the central spindle but the mechanism works very well. The case is Air Ministry (A. Crown M.) embossed with a 1940 date.  This is the type of measure used for marking out foundations/buildings and could have been used in RAF station building and runway construction work among many other possibilities.  Stock code M20482.