The hole is big enough for a small paperclip to feel it and go into it. The action of the shuttle sliding across the outside of the hole "glazes over" the hole to make it nearly disappear on the outside. Use your pin or needle inside of the hole (felt removed) if you want to, but a hook in a small wire (paperclip) may find the hole quicker. A bent pin might work too, but I usually use a small paperclip that I have on a shelf near my bench. You're just wanting to "feel" the indentation with a tool to zero in on it's location. Then, you can attack it from the outside to open it back up on the crusty ones.
CD in Oklahoma