That type of sight is an occluded Eye type. Similar to what we see on the Star Wars guns. The dot is provided (mainly) by the fiber optic piece in the front bubble. It would illuminate the dot inside a blacked out circle. Your other eye would provide the target image and the two would blend to give you your sight picture.
In case you're frying your brain trying to figure out how you're supposed to shoulder this thing to fire.... notice that back in the day when it came to close-quarters combat, "pointing and shooting" was the technique to go for. Everyone would train point-shooting, to be able to instinctively raise their gun and shoot, hitting the target, as easier as one can point their finger. (try pointing your finger at something. You don't even have to look down your arm to "aim" your finger, correct? That's the concept.) So, with that in mind, you can see this gun was designed around this old technique!
Thank you for the history and review. I know at some time in my life I owned that special weapons magazine you show in the video. I always look forward to your videos on the forgotten and rare in military history. Thanks again.