Shower handle stem broken

The handle for our shower fell off and it looks like the plastic part just inside the handle suffered plastic fatigue. The serrated-shaped top 'disc' sheared right off. We've looked all over these parts and can't find the manufacturer name and need help identifying the plastic part to replace it. Anyone know where we can purchase a replacement part?


Here are all the elements of the shower handle assembly:
Attachment 17494

Here is the plastic part which failed (top sheared right off):
Attachment 17486