I think it's called "adding lightness"
Same thing with both of mine, that's on a '72 car. I'd bet most UK cars which have had daily use through winterat some point in their lives will be in a similar state unless someone had made efforts to rust proof the parts, but they are in the "out of sight" category. And not easily checked, I bet the average MoT tester doesn't even know how the belt is secured.
I think there is a slight fail-safe aspect though. The main section where the bolt goes was roughly 3"x1.5" and thicker on mine, probably 3mm as opposed to 1.5mm of the rest of the anchor. So it would have to pull through the fibreglass sill almost in shear for complete failure and if you had that much force across your chest it probably wouldn't be the main thing on your mind.
Brian