That was one of the original (if not THE original) handgun scopes from waaaayyyy back! Dates from the late 60's/early70's or somewhere around that time.
If memory serves the base clamped onto the rail using a large thumbscrew. Finding a base could be a problem, may take a lot of searching.
Also, they were fairly popular for a short time for mounting on the barrel of top-eject Win model 94 lever actions ahead of the receiver. That was the original "scout rifle" setup long before the term scout rifle was coined.