The specifications of the picture-taking lenses are Yashikor 1:3.5 f=80mm and Yahsinon 1:3.5 f=80mm, respectively. Furthermore, the viewing lenses are more different with Yashikor 1:3.5 f-80mm and Yashinon 1:2.8 f=80mm. On the view/focusing screen, the effect of the differences is minor in my eyes. I feel that the screen behind the Yashinon 2.8 lens is slightly brighter in dim light situations. There is another obvious difference between the two cameras in my images above. Can you tell? A good thing is that Butkus has the manuals for both cameras free online. These manuals also show the variations of Yashica 635 with Yashikor (the grey-cover manual) and Yashinon (the yellow-cover manual) lenses.
I have another blog on Yashica 635 and its 35mm film adapter.