I got this with a B&J Orbit monorail a while back and was going to throw it away -- however, if someone here wants it and knows what it's worth (if anything) --
Says Simmon Omega Copy Attachment on the front of the folding cover -- I'm not sure if this goes with an Omega enlarger, fits on a monorail, or is part of a copy stand for 4x5. The target piece / plate has a glass negative holder with target marks for negative sizes from 4x5 down to 35mm.
Picture attached -- shipping would be between $8 and $12 conUS. If you want it or you have more information about what it is, let me know.