I should have clarified… 2.5" come in multiple forms, the common one and this not so common thing you found, which is called U.2, and is basically a cabled version of M.2.
Please ignore all U.2 or something called SFF-8639 – it’s just wrong for everyone here.
@Henry_McLeod – see above about U.2.
I moved this stuff out of the other topic as to not confuse newcomers.