I don't know why I never noticed this before, but it seems all the front derailleurs I have used
have clamps that do not fully-tighten. (That is, the two clamp faces at the securing bolt do not
meet when mounted).
Why is this?
Having a section of the securing bolt exposed like that, when mounted, makes it vulnerable
to the elements. Also, the clamp halves not meeting flush probably weakens the mount, at least theoretically.
The reason this is important to me is because I just bought a NOS Suntour X1, and when I
got it home, I measured it. The width was 28.6 (1 1/8"), as indicated by the seller, but the front
to back dimension was 26.9 (1 1/16"). To achieve the 28.6mm front-to back on the clamp, it must
remain somewhat open. Now I feel bad because I had accused the seller of being wrong about
the size.
I felt that there was no way that a clamp would be designed to not close fully.

Paul

If both ends do touch, it might not be tight enough and you'd want to use a shim. No need to worry about the bolt being exposed to the elements, they hold up quite well, IME.

+1 If you are concerned about the bolt, grease the threads before you install it. This is one bolt I've never had fail.