The mudguard. Drill a hole in it = stress riser so more likely to crack. PDW guards do come with a clip that allows you attach to mounts like that without drilling.
Pretty sure I just used the normal brackets and zip ties on my Inbred where there wasn’t even a threaded hole.
Cheers ah that's a pain. Maybe some zip tie bodge...
What often breaks? The mudguard or the stay?