9 times out of 10 the answer is in the form of Lechler numbers, which mean absolutely nothing to those of us in the USA.
Innocenti was not exactly great about releasing paint codes or paint chip books although for a brief period in the late 1950's into the early 1960's, they actually were.
Thanks to those, here we have the PPG formulas for all the codes released during that range.
Thanks Jon Kroge at Jet200!
See them all here. The images are links so click on them!