I checked my 1991 Horseman catalog to see if I could get a part number for you. That catalog shows the FA and its accessories. It does not, however, show any adapter or folding reflex viewer. In 1991 the only reflex viewer they show is the 6x9 reflex viewer for the 6x9 cameras.

So, i'm not sure if the one you have was made before or after 1991. In a related post it was indicated that a viewer like yours may need 3 parts but I don't have any more info:
Part 1.The hinged assembly fits hinge points on the back of a horseman and allows one to swing the viewer away from the camera to use a loupe. Part 2 is the reflex viewer body which fits onto the the hinged assembly (Part 1). Part 3. Is the bino loupe hood which fits Part 2.