The radius steel section with the two "mounting holes" looks exactly like a side-mount scope base would- and I suspect if you snugged it up against the side of a typical 1-1/4" or so receiver, it would conform to that radius. Along that same logic, the front "ring" would be over the centerline of the receiver. If it had a matching similar ring at the rear- and didn't have those indexed screws, it would most certainly be a side mount. Further, the flat section is attached to the lower with thumb screws- again, suggesting a side mount where you could remove the section containing the scope. I've got Williams side mounts that look exactly like that.
That rear square head bolt looks out of place. I'm wondering if there was, at some point in time, another ring attached there, just like the front. Wouldn't explain the indexing screws though- but otherwise, it would be a side mount scope base in that configuration.