Okay found it. In the December 2013 Federal Register, directly prior to the CFR revisions.
Where the mariner holds a management-level
credential, and fills the position as
mate, and the position meets the
definition of chief mate found in
§ 10.107, then that service will be
credited as chief mate.
Maybe JD Cavo can weigh in on how it panned out in implementation. The wording is also found in the preamble to the CFR’S, I knew I wasn’t taking crazy pills. Way I read it, if you want CM time you need a Management Level STCW