That's cast steel made in Carnegie, PA (Car-ne-gie, stretching the first & second syllable, then pronouncing the last one quickly - is the correct way to pronunciate the late Andrew Carnegie last name FYI :)
Just outside of Pittsburgh, PA... Very close to where I live nowadays... Most of those great mills of the thirties are torn down now but, a few like the US Steel (Now called USX) Edgar Thompson & the Mon Valley works are still around and of course, there's still the Union Electric steel mill in Carnegie, PA that's still producing, AK steel, Wheeling/Pitt steel in West Virginia just down the road about forty minutes from here... There are probably more around but off the top of my head I ca'nt remember... I know there's a few pipe mills on the south side making alot of stainless,one of them is an Italian firm that starts with an "M"...
I know there's more but, my memory the way it is lately, I'm lucky that I remember the ones I mentioned :)
Anywho, that's definitely US steel from the old Carnegie works, and more than likely a semi killed cast steel that when compared to the steel making technologies used today - were as Jeff mentioned, incompletely deoxidized but nonetheless, the best steel around in those days except for maybe some German steels.
That's all I can remember so - I'm done! Maybe "Brande" will chime in since he grew up here in these parts and know a heck of alot more than I could ever know about steelmaking history in the greater Pittsburgh region of South Western Pennsylvania. :)
Heck! I'm just a city boy from Manhattan turned semi-redneck "in the burgh" thanks to listening to bands like the "Poverty Neck Hillbillies" and other halfway decent country bands these days!!! That's right!!! This cityslicker has almost turned into a country boy :) :) :) The key word here is "Almost" :)
Let's hear it for my childhood favorite: "The Man in Black" - Johnny Cash!!!
Respectfully,
Henry