Well, I answered my own question a bit I guess. Apparently you clamp a file in it to dress handsaw teeth. Does anyone have practical experience using one? Here's a good link about it.
BTW, these came out of a homestead in interior Alaska, including several cabins built of local logs and hand-milled lumber. Circa 1948.
Man, that cross saw manual is fascinating! Makes me want to pull a nasty one out from a yard sale, clean it up and sharpen it! I definitely will keep a eye out for one of those saw filing tools you scored.