"Iron" and "steel" are very, very old words and do not mean the same today as they did in the 19th century.
Strictly speaking Iron is an element, like copper, gold, neon and oxygen.
The nearest READILY AVAILABLE thing to elemental iron is a common nail. Not a concrete nail, not something meant to be driven by some powered machine, just a plain ol' 10 penny nail.
Steel in olden times was iron with up to about 1.7% carbon added. Like wrought iron (which has maybe 3% by volume of slag in it) that has been deeply casehardened.
Alloy steel? Yeah, I suppose it might give different results in a stain. Just use nails or wash the oil off of your steel wool. Then wash off whatever you used to get rid of the oil.
Cast Iron is the element iron with maybe 4% by weight of carbon, present as graphite, and a couple % of silicon.
If you use some scrap item then you simply will not know what you have & no Dark of the Moon witchcraft will help.
Spark test? If you spark test for a living you can be astonishingly accurate, even distinguish one grade of stainless from another. If you don't make money doing it regularly, you may be just as remarkably wrong.
That is true of this P.I.T.A. metallurgist also (though I once found to my surprise that I could tell 321 stainless from 35%nickel 19%chromium alloy. That titanium in 321 makes a $#*! of a spark)