Here is the deal.
The early lids were thicker and allowed for a NPT fitting to be screwed into the lid directly and meet industry standards for thread contact with lid boss.
I have seen both with the presto.
If you look at the new presto they have a flat for the washer and nut to fit when it was stamped.
An option would be to machine out a nut with a taper on one side to accommodate the curve of the lid.
Really easy.
The later lids were thinner and required a nut on the bottom to keep the spout from blowing off.
Me thinks the steam boiler fitting standard is 3 threads contact but dont take my word for it.
Most now days are much more and way safe past standards.
Easy to figure the thread contact for any size thread pitch.
An example would be a 1/8 lid thickness divided by 27 would be 3.375 thread contact.