Hey,
You inquired about opening the Sennheiser PC 350 cups for modding.
Firstly, after pulling the earpads off the cups you aren't going to see all the screws,
because some of them are covered with black foam-tape. Unfortunately,
I don't remember which ones were originally covered, I've removed the
tape from my headset as I've opened them up quite a many times.
Secondly, there are a different number of screws in the two cups,
and the screws also come in two different sizes.
I opened mine up and counted them up;
In the left cup, which is the microphone side, there are 5 big screws and 2 small screws.
And the right cup has 4 big screws and 1 small screw.
The screws can be pretty tight at first and the driver enclosure can stick to the cup pretty hard,
just use caution and don't rush it, also beware the wires connecting from the enclosure to the cup.
I hope this helped.
Have fun with the mod!
-Jaawa