Fenrir 20 pin drives are motherboard revisions VA0, VA0.5, VA0.8, VA1. Please note some VA1 revisions come with a 21 pin drive instead of a 20 pin drive.
In order to find out if your console has a 20 pin drive without opening it, you can look for a screw at the back, next to the power supply socket. If you see 3 screws, then it's a 20 pin VA0. If not, then you need to check that the power and reset buttons are oval in shape and not round.
Another way to check, is to open the CD lid and look towards the back of the cd mechanism, to the gap on its right side. If you see a white ribbon cable running towards the back, then you have a 20 pin drive.
If none of the above help, then you most likely have a VA1 console and you need to open it to make sure your drive uses a 20 pin ribbon.