Cobb or Cell Phone Slot Prong and Socket (ball mount) piece and lock nut
This was remixed from a few different piece I found on here. This is a very common mount piece used for cell phones and Cobb Accessport car mounts and I was surprised I wasn't able to find this specific piece on here, so I remixed it from a couple I found. Included both STL and OBJ files
Lock Nut Remixed from:
https://www.thingiverse.com/thing:4942391
Slot Ball Socket Remixed from:
https://www.thingiverse.com/thing:5454776
Slot/Prong was remixed from:
https://www.printables.com/model/952217-wrx-cobb-accessport-mount
Can print with whatever material, but the socket portion for the ball needs extreme caution when sliding the ball in (hue hue) as the tabs are pretty fragile. Make sure the lock nut is already behind whatever portion ball mount you're using so you don't have to reinsert it.
PETG may be a little more forgiving for the socket tabs
The "Cobb Ball Joint Slot Mount6 LARGER" file will need to be printed with support. I recommend printing it with the slot tabs face down, then need to be cautious removing the supports not to break those tabs.