I removed the original register cover from my vent, the screw mounts were inside the duct.
I'm not sure if the fact that this is on an external wall has anything to do with it, but there was no bite behind the plaster when I drilled that pilot hole. It's possible I was hitting the brick. (The hole to the left of the duct is where I attempted to drill a pilot hole to install an updated 3 way register)
Typing this out I realize I could probably go down in the basement and confirm my suspicion that there's no stud there.
If these types of registers where the mounting holes are inside are still manufactured, and if so under what designation?
Alternatively how would someone go about installing a new grille in this setup?