I believe if you google the J. Adams Sheffield there is a little more history that can be found but it sounds like J. Adams Ltd bought up few of the names, including Sheffield, and make them.
Here's a 1989 NY Times article about how the Sheffield manufacturing has survived despite a decline in the industry, as a whole. Maybe useful, maybe not. I think this would preceded what I think was a purchase by J. Adams, who I think currently owns Sheffield according to the Sheffield Knives website, though maybe that's just a rename of the company.
From their website:
"
J. Adams Ltd (Sheffield Knives) is a family business which has been making good quality knives for six generations; we are the parent company for three other old established manufacturers
F.E & J.R.Hopkinson Ltd, established 14 June 1944,
John Nowill & Sons Ltd, whose corporate name was granted by the Cutlers Company of Hallamshire on 27 April 1700 A.D. and
Austin McGillivray & Co."
https://www.nytimes.com/1989/04/15/business/sheffield-knife-maker-beats-the-odds.html
About the Green River specifically, I believe it's an amazingly old design and I think it came up as an English variant of the Kephart design, though the Green River is older than Kephart so it could have easily inspired the design from whomever made Kephart's knives (the company alludes me right now, starts with a "c"). I really like the knife, I gifted it away but I have been running through ideas of how it would be nice to re-handle them, or try to find some blanks and use them for some semi-custom knives. The blocky handle needs a little work when new but it's pretty quick with a rasp and sand paper to smooth it out. Reminds me of some of the latin machetes that are a little roughly finished but after some slight personalization, very nice package for the money. But, you probably knew about the quality of the knife already having owned a few. I think these replace the utilization of morakniv for me, a budget bushcrafter that still works really well.