There's an interesting video posted by Kosmo Foto discussing the KMZ Start, an SLR camera made in the Soviet Union starting back in 1958. It's less than 10 minutes and you can find it here: KMZ Start. Also at this link in the description of the video is a link for further reading on Soviet cameras. This video is apparently the first in a series of videos to come looking at camera designs over the years. You may want to consider subscribing to the channel.