Beeswax

[Mickey the Finn]

Looking up "Cutler's resin" on Wikipedia may or may not be helpful. I can tell you: some of the "voids" on a few buffalo horn handles currently in my care have been filled in with some sort of dark grey/blackish and semi-solid mystery substance having a matte appearance and a softish and non-crumbly texture... Sort of like what I suspect might be had with a bit of charcoal and ashes kneaded into a bit of toilet bowl wax ring. I could be completely off here; it's just conjecture on my part.
Locating a tree which has been bleeding pitch which one can harvest is one thing; the beeswax might be trickier nowadays, unless your local community has a "Farmer's Market" on weekends in the parking lot of the shopping mall, or a Beekeepers Club which you can contact.
Charcoal can often be found in areas frequented by people "on or near the street". Or you can burn your own. About six and three quarter hours ago I was sawing up Mexican red oak and American white oak pallets to use as firewood... I also have numerous small containers and plastic baggies of sawdust which I've collected...ebony, ipe, wenge, various "mahoganies", rosewood, etc. (It's all MINE. I won't sell any. You can't have any); it's all potentially useful, depending on what you're trying to patch up/fill in. Collecting and making your own filler material is often much cheaper, and may yield better results than a can of some store-bought stuff. The unfortunate truth is: as society around the globe becomes increasingly urbanized, the path of least resistance may be straight to the "do it yourself" superstore down the boulevard (instead of to the pitch pine on the slope behind the superstore).
Your mileage may or may not vary.