25.8mm was a TTT clamp size, was considered analogous to 26.0 in every shop situation I can remember.
One can put an Allen key into the slot and then twist for a bit of expansion of the clamp, but too much force could weaken the stem.
Here is a picture showing said Allen key in place as I reamed out a 25.4mm stem for a 26.0mm handlebar. Note that this stem has a lot of meat in the clamp area (it's even available already sized for 26.0mm).
Also note that this sort of reaming procedure is self-aligning, and that these Origin-8 (Kalloy) stems are carefully sized to just slip into a French steer tube, even as they measure appreciably larger in quill diameter than any real French-sized stem from back in the day.
...with logo polished off and installed on a 26.0mm handlebar:
