If ever there were a goggled 21, I should think it'd be made as with the goggled 35s -- goggles attached. These are clearly made to be removed, which is right for the Summicron DR as you want the goggles on in the near range and off in the far range, but excess for the 21 as you want the goggles on whenever you have the lens mounted since their purpose isn't to adjust the rangefinder for near focus but to change the magnification of the viewfinder.
And if there were goggles there'd be no need for a separate finder.
These items weren't meant to go together, one is a 21mm external viewfinder the other is a set of goggles from a 50mm Summicron DR