I had two types of reflex viewfinders for screw-mount Leica, both non-Leitz : one was German, the Megoflex by Meyer & Co, the other American, the De Mornay-Budd. Frankly, their use was quite complicated, their size prohibitive and their exactness approximate, to say the least.
Here are some images, first of the Megoflex. The unit with folding cover-viewing case, was linked to the Elmar lens' front ring by a kind of arm, supposed to follow the distance setting. I had the two versions of the Megoflex, the second one having a different system of locking to the camera's accessory shoe.
The De Mornay-Budd was similar in concept yet more "modern" in design. The "cam" linking to the Summitar's front ring could be disengaged and turned 45º to the side.
Megoflex et De Mornay Budd - a copie réd..jpg 44.13KB 4 downloads
Megoflex - f copie réd..jpg 51.92KB 4 downloads
De Mornay Budd - g.jpg 51.97KB 2 downloads