MONSIEUR X

Monsieur X was a label used by several American department stores in the 1950s and 1960s to denote that the garment was a copy of a Dior design.  Stores like Macy's and Ohrbach's would buy a model from the couture house, and use it to make copies that were very much like the original.  Some stores even had fashion shows in which the Paris original was modeled alongside the copy.

There is also a Monsieur Y label, which denotes a Jacques Fath copy, and a Monsieur Z label, which is Givenchy.  These garments were not cheap copies, but instead were of a high quality and were quite expensive.