I looked inside before posting. The holes are about 3-4mm diameter.
There is nothing that I can see that looks like a screw for any head
or bit. It's just a darkness, even with a flashlight shining in. If
I try to probe the opening with a small screwdriver, it goes in about
an inch before encountering something, and in one of the holes, the
thing being encountered isn't a surface that stops the screwdriver so
much as it diverts the screwdriver to one side.
I'd like to post a picture, but I'm using Google Groups to access this
forum. What is the next best way to post a picture? (I don't do
social media, and I've learned long ago to be anonymous on usenet).