How do you unlock the altitude mirror adjustment wheel on an A 10A bubble sextant?

During WWII the bubble sextant found constant use on both sides---one of the best known and most frequently used designs of bubble sextant was the ANSCO A 10A aircraft bubble sextant...there is a small hand wheel or rotating knob on the bottom of the instrument which, when rotated, causes the the index mirror to likewise rotate by way of a gear train----but apparently this knob can be locked by some kind of detent or other means so that it cannot turn.....can anyone out there shed some light on this?