Glad to finally see a picture of one on a 99. I will continue to look for one now that I have some sort of identification. I have thought about just plugging the holes and using a peep sight but I really wanted to use an optic for coyote hunting. I don't want to swiss cheese the receiver anymore for another mount. I had a smith offer to plug the holes and Tig weld them and reblue since the original bluing is pretty worn but I didn't follow through since I was trying to identify the proper mount.
My guess there are some of these around but removed so long ago that nobody knows what they or how to identify them. I would be curious to know the angle of the base screw. I assume they had to have some sort of jig or least installation instructions to properly drill and align the holes.
I have an old scope like yours from another gun. I wonder if you could take one of the Picatinny rail style modern rings and modify it to work? You would have to radius the bottom side and adjust the crossbolt screw angle. Just spitballin here!