Help identify spare part after CX assembly

Looks like a deraileur hanger to me.

bingo

Begs the question of just what does the rear derailleur look like right now. How is it attached? That might just be a spare hanger that was sent with, if so that is a handy thing to stash away for when you need it. Does not take much force to bend a hanger and at Tri races on over crowded bike racks stuff happens. I had my front derailleur pushed in a few weeks ago. Took me a few minutes to figure out why post race I was getting chain rub and poor front ring shifting.

Ahaa....thanks! Having "leftover parts" is always a concern! The rear DR arrived installed and after checking just now I see the exact same hanger being used. So it appears to be a spare. Is the idea that the hanger will take the force instead of the frame or DR? Thus a much cheaper fix?

Yes. A new hanger is much cheaper than a new frame. most all carbon and aluminum frames have replaceable hangers. Steel frames can be bent back to the proper alignment.