Unless you're prepared to replace the whole beam, I wouldn't go cutting into it. In a house that old it could become a nightmare in a hot minute.
It sounds like someone applied Wood Hardener to it, at some point. "precisely for the reasons stated above".
I would fill it, and patch it.
Put a piece of Plywood over it with quick setting glue, so you don't cause more damage. Make it 3" bigger then the hole to be filled on all four sides. Seal the top, bottom and sides of the plywood. Drill a 1" hole above the patch board at an angle into the void to be filled. Get a long stemmed funnel that will go into the hole. Fill the void with one cup of two part epoxy mix. Stop after one cup, let dry for 24hrs. That will let the epoxy soak in and fill any possible leak points at the bottom of the void. Come back and top off the hole until filled. The patched area will be stronger then all the wood around it, and it will be completely bonded together.
"Error of opinion may be tolerated where reason is left free to combat it"
Last edited by Cowboy18; 11-20-2017 at 02:06 AM.