There were several different versions with flat/round lenses even some green dots later on
The dots were rarely round and not much use beyond close range but they fulfilled a requirement at the time until Hakko red dot and Tasco aimpoints came along , they saw limited and massively over emphasised military service, hence their popularity through the late 70s and early 80s.
They are virtually unbreakable apart from the lens and without caps will still attract over £100 on the bay of fleas
HTH
"But we have our own dream and our own task. We are with Europe, but not of it. We are linked, but not comprised. We are interested and associated, but not absorbed."
Winston Churchill 1930