Any guidance on how to do this? Insect damage. I initially thought to fill the holes with black epoxy. Is this OK?

It does work. However if you have some form of horn dust to mix in it would be better. Be careful, just sanding horn on a piece of sand paper will give you broken abrasive parts too. They do show up. I have used plain Acra-glass liquid for this process with good results. Just go slowly and do not be afraid of making many applications if you have multiple holes. You may not get all of them filled the first time, or they may be in a wrong angle to best fill the hole the first time around. Mix small batches for best results.

Genelang:
Unless the holes threaten the structural integrity of the plate, why mess with them?

These holes are part of the material's charm and history. They are actually "cool" to a lot of collectors.

But many American's want everything to look perfectly "new" and pristine, so do what you will.

I mix Epoxy carbon black, powdered snuff and touch of graphite powder to match the Horn or other buttplate material. With horn. generally leave out the carbon black unless the Horn is very dark.

Brownells dyes made for their acra glass work well.

I'm afraid the damage MAY be structurally significant. The gun has plenty of charm. I'm not trying to restore it to like new condition.

Sometimes old damage is ... well just old damage and needs fixin.