I agree with Hakone in so far that the staminodium of his flower is quite deeply notched and I can see why he sees callosum influence there. But I have to agree with Rick as well, the staminode in the appletonianum group is so variable, it is hard to base an identification solely on this feature. From what I can see in his pic so far, Hakones flower is very heavy on the appletonianum side, at least.
I googled a bit and found a few pics of "x siamense". They show some variation but in most cases the callosum influence is visible, and that's the way I remember the x siamenses I've seen at shows.
Also, in addition to my previous comment, some comments to those pics on vendor sites indicate that the plants offered are seed grown, and that makes them Siamense not x siamense. Which is good.
Still, x siamense would be something for the treasure hunter and Siamense for the trash box, in my opinion.
Cheers, Carsten