If this baby stay tang is anything like a chainplate, I found some moist balsa and rot around my chainplate slot last fall. I dug it out with a succession of tools (pieces of clothes hanger wire mainly) until I got what felt like all of it. Amounted to an inch to inch and a half gap around the slot. I filled it all up with thickened epoxy and then redrilled out the slot.
You might want to get some longer probes than an allen wrench to see the extent of your problem. If you've pretty much got it all, maybe filling from the slot will do. We'll see what others say.
2000 Beneteau 331
Northern Chesapeake Bay
Ambition is a poor excuse for not having enough sense to be lazy ~ Steven Wright