Keith, if that K&H was a good file, go on and make something with it and test it. It's the only way to know for sure.
Troop, I've heard conflicting reports about Nicholson's... saying they are either 1095 or W2... don't matter much to me since either steel can make a good knife.
Jake, I never used a Napa file, but if they're good and last a long time, go for it. It MAY be that they're made in the same factory as XXX-brand and just stamped with the Napa name. I have no info on that, but that sort of thing happens all the time.
In short, although I'm no expert, I will say if you have a file you've used hard for a long time before it went dull, there's a fair chance it's made of good steel. As opposed to the cheap India/China files from the dollar store that last about one day (I believe they're just case-hardened junk steel, that's why they wear out so fast.)
One test is to break one, before doing anything else to it. Clamp the first inch in a vise and snap it off. Or just whack it flat on your bench. It won't take much to break it, because it's very hard and brittle. If the surfaces of the broken parts look like cast metal, guess what? It's either junk steel or was badly HT'ed. Nicholson files, when broken, have a very smooth clean look on the busted surface, barely any "grain" showing at all. I believe this is a good sign; good steel that was HT'ed well. I don't have a microscope, so that's the best description I can offer.
I'm NOT a steel expert, this is just what I've found out by working with files and learned from what I've read. Bottom line is, make one, deal with it the best you can, and beat the snot out of it before you finish it up and sell it.
I'm not saying a file knife is the best knife, but I've made several file knives that beat factory knives hands-down in every area of performance. As I get my HT set-up going, I will endeavor to improve on that, whether I'm using Nicholsons or CPM3V or whatever.