After talking with those more knowledgeable than myself (in Chester, CT), I retract my impression that this Masi in question has pre-formed seat & chain stay "junctions". They are in fact shaped using a flat file (rather than more often used round file.)
I incorrectly assumed this was the case.
There WERE Columbus stays that were pressed into shaped w/o filing by using Marchetti & Lange or Bike Machinery machine, which uses mega force & squeezed and shaped the Columbus stays into a shape that is recognizable... Many Eddy Merckx bikes, as well as others, used this method.
But apparently even these were not pre-formed by Columbus but instead done in-place as part of frame assembly prior to brazing... It would be interesting to see the gadget that accomplished this labor saving step.
Reynolds stays could be ordered pre-domed and slotted, but not Columbus.
Dale Brown
Greensboro, North Carolina USA