Just took another look at my III.
There are two holes, on opposite sides.
One has the two ends of the clip and the other just exposes a little notch (in the same clip) which is for lining everything up on reassembly.
The picture you've shown above looks to show the notch...Is there a hole on the other side?
If yours is the same as mine then pushing in the clip ends should let you take the plastic cap off at an angle - I didn't mention that earlier.
Also, it was a little tricky...I just removed mine to double check.. It took a bit of jiggling and poking.
I can also say that behind that circlip is solid metal so don't reach for the hammer or anything, but you should be safe enough putting reasonable pressure on there.
Also there are little recesses in the side of the capsule, at certain points, so the rotate/try again thing is a good idea.
There are points at which the clip ends just won't depress.
Hey Nola...Yeah, I could do a video if it helps.