Looking at my Makina 67 I find also this plane glass window, but I also see a lens directly behind this plane glass.
I made an image with my mobile phone - it needed some setup (my desk lamp plus a torch) to make it visible, but I think now it can be observed in the image attached:
I am not quite sure if this lens is visible in your image (but as I found it depends on the light if one can see it in a photo).
Edit: Looking more in detail at your image it also seems that the cover metal sheet is bent (on the backside of the camera at the position where it contacts the cold shoe) - if this is the case it may be a more serious defect than only the viewfinder glass.
Best regards
Heinz