I think you may be misunderstanding that older post. I believe from context that the author of that post capitalized the word “is” for emphasis, rather than stating that “IS” is a stamp. He will be able to clarify for us. Good luck on your search!
Interesting that pipe stamping, which is supposed to identify and clarify, often is a mystery in itself. Be "nice" if stamps were simplified, uniform between brands, and in plain language, with numbers in a standard format. I think competition between brands enticed pipe makers to keep the stamps coded. Too bad. Not enlightened over the long term. Now they obscure rather than clarify information.
The main intent of code stamping, on most of the older pipes at least, was to identify and clarify information to the production and distribution teams. Those guys knew exactly what information was being conveyed by the stamp.Interesting that pipe stamping, which is supposed to identify and clarify, often is a mystery in itself.