Regarding straight sided files, all the ones in my toolbox are of
the type “grundy old files for your year of looking,” I prowl
garage sales and flea markets looking for old tools of all types
since they just don’t make a lot of the older styles, or they are
made poorly today. Old and grungy is okay as long as the file is not
damaged and will clean up and cut properly.
I have also modified a few files when necessary by annealing,
grinding to shape, chiseling in the teeth, and then reharding. It is
a lot of work but if you enjoy metalsmithing, then you get a special
satisfaction of making your own tool to your specifications. Just my
two cents worth.