Serial number might help. It looks like a time trial frame to me but I don't recall seeing a similar frame previously. Aluminum fork? That would put it 1989 or later. If the decals are original, I would guess 1989-1991.
I am guessing that it's not a production Cannondale. Construction looks similar though. I can't see the dropout area in the picture. Cannondale used cantilever dropouts on their road frames from 1989 to 1996. Cannondale was looking to get into European cycling in that era. I know Banesto switched from TVT to Pinarello at some point in the early to mid 1990s. Perhaps Cannondale built them some prototypes to test.
I honestly think it's a Cannondale prototype. I don't see anything in your pictures that is inconsistent with the way Cannondales were made. A friend that was on a USA Cannondale team told me that he used to get frames that were not in production, so we know at minimum that frames exist that aren't in the catalogs.