John Moss and I put it together about 15 years ago. There were two purposes. One was to attract more people to the website and the other was to provide standard company names that the ATF accepted. We had a problem with the ATF disapproving import permits from foreign collectors coming to SLICS because they considered the names of the manufacturers on the forms was incomplete or incorrect. We worked with the ATF and they contributed to the list so if someone was filling out a Form 6 NIA they could just cut and paste the name from this headstamp code listing and the ATF accepted it.
We tried to update it for a while but it became a huge job and the list did not include rimfire or shotshells or much else that should be there. A talented and computer savy collector built an Access database that provided the capability to expand the list to rimfire, shotshells, large caliber and other catagories and vastly simplified the input. We tried for a number of years to get volunteers to take over the various sections and someone to coordinate the whole thing and had a number of false starts but I believe there has not been an update for about 10 years, perhaps longer. The database also allows images of symbols, foreign characters and even images like birds or flowers to be included. It is really a group effort to update it and keep it current, and requires some system to make sure the entries are correct. The original and the database version both included the reference for the source of the identification which is critical.
When the list first went up it was copied all over the internet. Even PPU had a copy on their website, and the ATF used much of it for the database they use for their automated Form 6. I searched one day about a decade ago and found 30-40 copies on various websites, about a third of them actually credited the IAA website!
There you have the history. I would love to see someone put together a group to care for and feet this listing.