I’m guessing a key hole bit.
Freud 25/64" (Dia.) Key Hole Bit with 1/4" Shank (70-104) by Amazon.com Learn more: https://www.amazon.com/dp/B00004T7JX/ref=cm_sw_em_r_mt_dp_GVETW667Z98B7XQ2002Y?_encoding=UTF8&psc=1
My dad's birthday is coming up, and he and my mom bought a lake house this year that they're still getting just right. I thought I could make a key rack (largely based on Jay Bates's design) for the doorway. The major deviation from Jay's design is that I routed keyhole slots on the back for hanging instead of having unsightly (imo) screws visible from the front.
Here's a link to Jay's key holder video if you're interested.
This is the router bit that I used to make the keyhole slots on the back (sorry, forgot to get images uploaded of those). Worked very well with a plunge router, and I think it looks way nicer that way.
How do you figure? I regularly make keyholes in stock less than 1/2" thick with one of these.