For those who like such, there appears to be a Topcon 35mm prototype, called the AM-1, no lens, looks like it has a custom made viewfinder, on the Goodwill auction site.
Why do you think this camera never made it into production? There were many versions of the Topcons and other cameras which were used for medical and scientific purposes. They were usually sold through a different channel than the regular cameras.
The AM-1 wasn't the same camera as the AM it was made by a different company and re-badged under various names including Topcon, it was K mount not Exacta mount.
Strictly speaking it wasn't a prototype rather that only a few were sold badged as Topcon cameras. As a camera brand Topcon didn't last long after switching to selling 3rd party K mount cameras which weren't as well made as their earlier cameras and the AM-1 coincides with their leaving the camera market.