I have cut a few tubers apart in my day, both accidentally and purposefully.
The tissue seems devoid of structure, so I don't think they're bulbs. The
scale leaves argue against tubers. So this leaves me with corms or possibly
something else called a "stem tuber" which is just a thickened underground
stem, and sometimes similar to a rhizome. But a corm seems to be more
appropriate.
I just looked at a few other reference sources, and one (a basic botany
text) classes the "tubers" of a potato as a tuber, even though it clearly
bears buds in spiral arrangement and would seem to be more like a corm.
My prime morphology reference I mentioned in paragraphs 1,2 classifies
the potatoe as a stem tuber.
B