I've just released another ThinkPad guide, this one a list of interesting and unique models. I'll likely add more models to that list eventually like the highly customizable, PowerPC based ThinkPad 850, but for now we have a decent list with a few models, and even one so rare, I can't find any real information about it, at least any documents in English.
Of course, it doesn't help that, despite never posting (or even having an account) on the ThinkPad forum, I've been flagged as a spammer when trying to make an account and have had my IP banned. Why? Don't ask me. Maybe they're part of a vast conspiracy to cover up the existence of the ThinkPad X31 10th anniversary edition, or maybe I'm just unlucky. Either way, I sent a request to support asking what I can do about the ban. I'll update the page with more information on the X31, probably in a few days if I can take care of this ban and register an account to be able to use the forum. For now I'm going to leave it here.
If you know anything about the X31 10th anniversary edition, please send mail to me at mckinley[at]mckinley[dot]cc and I'll have a look. Thank you.
Update 2020-12-07: Updated link to the ThinkPad Unicorns list as part of blog re-organization.