I was a bit curious and did some internet research just now. Any of this info could be in error, wrong, or otherwise false, as it does merely come from the internet.
Remsport Manufacturing (not RENNSPORT) is in Ludlow, MA.
Remsport appears to be owned by another corporation called RCS Diesel Services Inc. aka Ron Chiasson & Sons Diesel Services. This business was started in ~1983.
Ref.
http://fflgundealers.net/remsport-mfg.html
Another website, 1911Pro.com, had someone comment the 1911 forgings used were from an unnamed South Korean company. This would make the forging a material supplied to/bought by Remsport, which then machines the forgings into a 1911 frame.
Ref.
http://www.1911pro.com/forum/viewtopic.php?f=7&t=2494&start=10
Steel is a material which has a primary raw material of iron. Where the iron originates from was beyond my research, but here's a possible source of more info:
https://en.wikipedia.org/wiki/Iron_ore#Available_world_iron_ore_resources
To summarize:
iron/iron ore comes out of the earth from a variety of locations around the world;
a variety of companies & locations around the world will produce steel;
a variety of companies may purchase steel and make forgings;
a company, like Remsport, or any other firearms manufacturer may buy a forging and then machine its own semi-finished frame, or may purchase a semi-finished frame. To what extent this info is company-confidential varies.
To say Remsport is "South Korean" may be more complicated.
When I say frames, I suppose this also applies to a 1911 slide.