these kinds of worms....

https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=646384#Post646384

horn is fairly largely low grade protein, and will be eaten by a wide variety of fauna...shed horns have to be picked up fairly quickly, before the creepy crawly things begin to munch on them. hard to say exactly how/which bugs get to the butt plates...but, it's quite common to see such damage - often on guns otherwise not so much showing neglect.

best regards,

tom

"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland