I think you are confusing the eyepiece with the viewfinder. The early Minolta SR cameras may have round eyepieces but they have plain rectangular viewfinders. The SRT cameras have slightly rounded corners in their viewfinder but the image on film is a plain rectangle. The SRT cameras were never made to have 100% of what appeared on film also show up in the viewfinder. A common explanation for this is that many advanced amateurs used slide film. When the slides were mounted, a small percentage of the image are would be covered by the edges of the mount. There have been cameras with what appeared to be circular viewfinders. These were primarily for scientific or medical use. An endoscope transmits an image through a fiber optic, which projects a circular image onto the focusing screen. If I need a camera which guarantees that 100% of what appears on film will also appear in the finder, I can use a Nikon F, F2 or F3.
I have a roll of film in a Minolta SRT 200 which was recently overhauled by John Titterington. It's a late production model with the split image focusing screen. It joins my large collection of SRT cameras.