You’re correct, there were some sort of plastic clips. Beyond that, I can’t tell you much about them except that the only reason I know that much about them is because they were usually broken by people who thought that pad was covering a compartment of some kind and pulled on it to open it. The best I can tell is the clips were made to work once.
If you’re looking for something to take their place, I’ve used Velcro and that seems to work very well.