One factor I don't think we've dived into very much is, what did Mountain Goat have access to / what were Mountain Goat using up until they ceased to exist (in name)? I wonder if there was a surplus of some tube-sets and they were renamed to something generic so not to alert the actual manufacturers.
Normally I could put something together rather better to do some more effective comparison, but I'm not so tickety-boo right now, and have left the harder work to whoever may feel the need to wade through these pictures.
Yes you are correct regarding the tubing that was used for the confirmed seven Caldera frames that we have seen so far. What we see, is a variance in the Bulge of the Seat Tube at the Bottom Bracket Junction in these frames. Needless to say, they are likely to be left over Mountain Goat stock, and renamed after Jeff's MG Company folder in 1995.
Anyway...... elmadu:
CA9609021, Molotov Red 16”, and your
CA9609046, Poopsy Blue, 20" have no Bulge:
While konaman1:
K09610031, Poopsy Blue, 18" does have a Bulge - Prefix notwithstanding to account for anything

!
I can add some pages from the Tange Catalogue to see if that helps with identifying any tubing - I should get the whole catalogue added to Archive.
Fill your boots:
Pip