Even more Trek MTB sizing tidbits:

It looks like Bottom Bracket to Top Tube Center-To-Center up until 1990, possibly 1991.

Bottom Bracket to seat collar in 1992, with the BB to top tube dimension still shown in catalogs.

And Holy Cow! Trek is still doing that Virtual Size bit. I dunno what to make of that.

I think I'm too old to figure out MTB sizing past the mid 90s, road sizing beyond 2002 or so.