Coated roof prism erector assembly. About 2" long. Slides into 1-1/4" eyepiece tube. Kept in place with spring clip.
Worked fine with my Televue 85 and 20 mm plossl for terrestial use, after getting the spacing just right. Now have a Leica 77 APO for terrestial use so this is surplus.
No box, no instructions, no maker id that I can see. Bought from my telescope dealer (perceptor) about 8 years ago. NOT recommended for medium/ high power use (no erectors are).
Have fun and experiment for $10 shipped anywhere in North America.