Cinelli LA84 x 3TTT stem troubles...

I've got a 3TTT stem with a similar looking arrangement (difficult to see from the photo): a small bolt screws into the stem and then another bolt screws into that.

It's also such a system, no idea why someone would design this...

If you are in love with the stem for this build and don't mind 'ruining' it you should be able to use a round file to enlarge the inside diameter. I've done it with a 25.4 up to 26.0.

Hmm, that could be an idea