That is correct. By the look of the photo of the bar clamp area it certainly looks like an MTB version at 25.4mm.
These are beautiful and pretty rare titanium stems, I have a very similar one.
Beautiful bike, I have one just like it.
FYI, the title states 91 although it’s actually a 1990 with a not so early serial number looking at the position of the downtube cable inlets. GLWS, worth every penny.