I decided to give the wood one more treatment with the oxalic acid. After reviewing my go to reference on wood finishing by Flexner I decided to follow his advice and leave the solution on until it dried, then rinse off with clean water and neutralize with soda water or ammonia bath. I used soda. Results are excellent. Now I have the walnut piece from 1893.

Oxalic solution dried on wood:


Cleaned forend:

Wiped with mineral spirits