I don't know what the rarest Matra road car is(various prototypes would probably make that cut), but the rarest(or at least least known) Matra powered racing car I know of is the Shadow DN7. It was basically(from what I understand) a DN5 modified to use the Matra V12 instead of the Ford DFV V8. At the time, Shadow Cars, Inc was considering switching to the Matra engine.
Jean-Pierre Jarier drove the car twice('75 Austrian and Italian GPs), the car having fuel injection problems both outings. Dispite it's potential, Shadow continued to use the DFV until the team folded in mid 1980. As a result of Matra and the UOP Shadow team going their separate ways(Shadow couldn't afford to by huge stocks of the engines), Guy Liger's team bought up the Matra V12s and used them with good effect in '76-78, and from 81-82, when they switched to Renault 1.5 V6 turbo engines(they used Cosworth DFVs in 79-80).
As for the DN7, only one was built, and believe it or not, was placed in storage(instead of being destroyed) for over 20 years when it was discovered in good condition in 2005 or '06.
Wikipedia has a photo of the engine area of the car in the Shadow Racing Cars article. And a F1 site,
http://f1-facts.com has photos of the DN7. On the overview section click teams, and find Shadow-the first photo is the DN7, and there's another photo under the car's profile.
Hopefully you'll enjoy the ultra rare Matra racing car, and hopefully someone will have some info on it.