The Stand Instrument No21 plus a few others were generally used in the instrument workshops in order to hold an instrument such as the MG theodolites and directors plus compasses, binos....., you get the picture, steady while you set about calibrating them. You could change the head to accommodate different instruments. The big one was solid brass and we had another brass adaptor for that for compasses
If you didn't have these special stands some instruments were sometimes such an akward shape that it was difficult to hold and aim. You could just put a director on the window sill while doing calibration tests........... But you'd only let it drop once!