Grateful, I hardly ever visit yard sales (my wife won't let me--she's afraid I'm going to bring home junk). And, I'm afraid that she's probably right--I like to accumulate things.
However, it's my dream to someday "accidentally" end up at a yard sale and find a rare, high-end, custom folder being sold for a song. Okay, you can say it now, "Dream on, toothed, you fool!"
Realistically, I hope to find myself in a situation similar to yours where I'm surprised with a folder that has been overlooked by others. It's the hunt (for that special folder) that is exciting for me.
Back to the topic: According to a knife collector's guide I have, the Western States Cutlery Co. started in 1920 in Boulder, Colorado. About 1950, the company dropped "States" from its name and their stamping was also changed to "Western USA." The guide does not say when the company changed its stamp again so one could only surmise that your knife may have been made as early as 1950 or as late as ____ ? I'm just quoting from the guide and don't really know the accuracy of this information but hopefully it will be a start for you.