That makes perfect sense! One of the lid screws was missing in my machine (luckily, not the lug - I replaced both screws with regular Phillips head screws, easier to work with since you can't see them). Perhaps the spring is supposed to push the lid holder lug up so the lid is easier to install, or just to keep it from rattling around when they are loose?
Maybe I'll list it on eBay as a genuine vintage B&O spring