>"How was parallelism checked between the seat tube
>and head tube and how was that corrected/dealt with?"

there was something referred to as a british bender; a long rod
that fit between two head tube cones telescoped a visual all the
way beneath the lower head tube lug and another 12-20 inches.
a straight edge leaned against the seat/down tube diameters
had to skim a certain spot to yield a "correct" or acceptable
alignment check. this was done AFTER the linearity of the seat
and down tube was okay-ed.
as with alignment tables, all this was also done when the frame
was in the tacked stages.