Thanks for the info, John - I don't need to mooch files off of you. It's kind of fun to find these, but the ones with the white tangs are precisely the ones that I like the most.
I don't know anything about heller, but their heavier older files are thick in cross section (giving enough room on the flat files to hammer and then cut bevels) and they seem to get the hardest, especially in thinner cross section. To the point that some are too hard, and maybe a bit chippy even at a straw temper. This one had straw color on it out of the toaster oven, but it's ungodly hard and would be better tempered back.
It's interesting that two different people going based on feel and what's observable would probably pretty quickly come to the same conclusion. These are probably water hardening steel, but they get this high hardness in an unheated soy oil quench - a very pleasant surprise.
This chisel was a 12 or 14 inch flat metal file, but most of the ones that i'm getting in large size have a horseshoe on them, and sometimes an obvious tooth profile and size that looks better suited for a hoof than steel.
One other favorite so far is a file that says "india" and has a picture of a palm tree. No clue what they are, but the spark from them is fine and intense, and the perceived fineness of the steel is slight better (and they come out just softer).