I'm not sure what it is.......but it kinda looks like a Phylrich stem. I looked over phylrichs website and didn't see it but that doesn't mean jack.
Can you take a pic of the back of the valve in the wall? Maybe you have an access panel,maybe not.
The packing nut holds that stemm in from what I see. In one pic you have the packing nut removed.
Do not try to rotate the stem counter clockwise.....I believe the longer spline is actually threaded into the interior portion of the diverter and it has a permanent type epoxy all installed at the factory. So if you try to rotate counter clockwise it could break that connection and you end up with the spline in your hand and the half the diverter stem still left in the faucet.
Try to rotate the stem clockwise. Use some force but dont kill it. The stem should pull straight out.......it does not thread into the valve.
If you have access from the back you might try heating that portion of the valve body to expand it......be very careful not to over heat if you do this. Just get it hot enough that you cant touch it . If you heat too much it may make it harder to get out.
Goodluck to you sir.