When they surplus that stuff the workers don't care and the equipment gets slide or worse, thrown around, knocking the feet off. And breaking off the knobs but that's a different story. In the surplus areas you'll find them laying all over the floors but they're usually lost by the time they get to someone that cares about the stuff.
BTW even the clip on feet aren't all the same. I pulled out a bag of spares today and looked at them and some only say "rear feet" in small letters. Other's have much larger print and much more text including instructions to release the clip and slide the foot off. They're also molded much cleanly. I'm sure that HP had them made by numerous sub contractors over the years.
PS I just went and looked again. I thought that perhaps the "rear feet" didn't have the notch for the folding metal bail but they do. I have no idea why they marked them "rear".