This is a finderscope or laser pointer holder for an astronomical telescope. It will clamp in an Agena Synta / Vixen style dovetail base. It uses 4-40 threaded inserts (McMaster-Carr part number: 99362A200) to put reliable threads in the mounting rings - especially at this pitch, I wouldn't rely on tapped threads to last very long. The finderscope or laser pointer is suspended in place using 4-40 thumb screws (I'm using McMaster-Carr part number: 98704A125).
I printed this at 100% infill for strength.