What are you going to install into your Porsche 911 Coupe (991)
(we need this information to establish the correct parts needed)?

Do I have a CDR31 or PCM3.1 System?

The difference of having PCM 3.1 or CDR 3.1 is the buttons in the units centre console.

If your system has CD/ AUX button and AM/FM buttons then you have a CDR 3.1/ CDR31

If your system has NAVI/ MAP buttons and a SD Slot then you have a PCM 3.1