The Collaborative International Dictionary of English v.0.48:

Montre \Mon"tre\, n. [F., show, show case, organ case.]
   1. (Organ Building) A stop, usually the open diapason, having
      its pipes "shown" as part of the organ case, or otherwise
      specially mounted.
      [Webster 1913 Suppl.]

   2. A hole in the wall of a pottery kiln, by which the state
      of the pieces within can be judged.
      [Webster 1913 Suppl.]