As short (race) drop brakes came into being, it was also the point at which Allen key mounting became common. So, while what you want may be out there, it might not be common.
Another approach might be to use an AK fitting front brake on the rear with a nut and try to find a long bolt for the front
As an aside, I'm intrigued that you're looking for race drop brakes for a Wester Ross, as I thought they were all touring frames??