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

During WWII the bubble tant found constant use on both sides---one of the best known and most frequently used designs of bubble tant was the ANSCO A 10A aircraft bubble tant...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?