On the back side of the carrier, note the tang where the focusing shaft is installed.
On some mirrors, the back side of the mirror is marked to indicated its position
relative to that tang. Note the big black marking behind the tang on this mirror,
along with the number 96 (not a clue what that is for) and the serial number of the
mirror under my right thumb. If your mirror is not obviously marked, use a marker
and outline the tang right on the back of the mirror, having it in the correct
rotational position is somewhat important.