Hello from Switzerland,
As I remember, Concept was the parts and accessories brand for Lawee Cycles in the USA, who were best known for their Univega bike brand. It could be that they created this "One Off Proto" as a showpiece to highlight the components available through the Concept brand and Lawee Cycles' parts distribution.
What's funny is that I don't believe that you have the only example of this "One Off Proto" model. I believe I saw a similar, or the same, bicycle in the archives of the Mountainbike Museum in Arnhem, Holland a few weeks ago. Jeroen is likely too busy getting the building ready for an official opening to answer detailed questions about one of his 700+ bikes, but I can try to take some pictures of his the next time I visit - perhaps in August.
Finally, since Lawee was based in Southern California, it could be that Kastan built the frame. Something about the rear dropouts reminds me of other frames which were built there.
Best of luck,
The Kid