I have 2nd and 3rd editions of Poyer's book. Page #100 has tables citing changes to receiver.
Your page #100 is page 96 in my copies of Poyer.

Is your book the 1st edition?

I believe Poyer was simply saying that the breech marking identified the steel maker of a Kragicon barrel.

Poyer was also stating the Chief of Ordnance Annual Reports mentioned the steel manufacturers used in a given year.

I don't believe a 'Rosetta stone' has been found to decode these markings, the code of which was once known. (Some things do get forgotten or lost or are yet to be rediscovered).