My daughter sent me this definition which she found with an internet search: If only one copy of a product is required, this type of production is described as "one-off".

Then I have found some uses of the term "one-off" on the internet, as it applies to Pontiacs. In each it appears to be used to discuss a product (in these cases a Pontiac) that is one-of-a-kind prototype, experimental, factory showcar, etc.

http://pontiacserver.freeservers.com/hpp575bon1.html (title)
http://www.firebirdgallery.com/72lot1.htm (second paragraph)
http://www.hotrod.com/featuredvehicles/96898/ (title)
http://www.poci.org/2004convention/conventionrules.shtml (Vote Class/category FX)
http://www.highperformancepontiac.com/features/0408pon_deptx/ (First sentence in article body)