Oh, right! That's different from the one I have.

For what it's worth, I've found some Idéale saddles used thinner rails than Brooks, so when mounting an Idéale I sometimes have to wrap something around the rail to get the clamp tight. That's not the same problem as OP is having, but perhaps related?

An alternative to filing is to add a something like a washer to take up extra space and grip, such as a disk of coarse sandpaper.