bergenk The side screw does have anything in it.
It has dry air in it :-).
In the old days, ZWO shipped the camera with a tube that contains desiccant tablets. The tube screws into that hole from which you had removed the sealing screw (your second picture).
It probably was not very effective, so they stopped using it, but unfortunately did not retool their camera bodies, but left the hole there, usually with the screw with the O-ring covered with a black plastic button. Who knows, perhaps that is how they factory dry the camera chambers, thus still need the hole.
Unfortunately, if you have opened that seal for more than a few minutes, you might now need to replace the camera's desiccant tablets in your camera chamber, since I don't believe they plugged the hole from the insde either (i.e., enough moist air could have degraded your existing desiccant tablet (watch for icing on the sensor)).
Chen