Is it chrome? Possibly just a sleeve.
You can chip out a little tile to look inside the wall.
Flat tip screw driver right on the edge and hard tap with a hammer.
You can chip a little off the edges. 1/8th of an inch at a time.
If you want to see more inside the wall.
Try and pull off the sleeve if you can.
I have never seen a stem with a slot in the end like that