Fourbore:  I am not sure exactly what you are trying to accomplish with such exactitude. (If that is a word.) I have been collecting and repairing and fixing and shooting Krags for years and have seen such a variety of authentic rifles and carbines that I don't think that there is any one exact tone or finish that isn't generally accepted as "correct" and "original" today. Heck, they're 120+ years old! Consider the extent that these firearms have been used and handled and stored and abused and all of the grease and oil and sweat and dirt they have accumulated over these hundred plus years. I have a huge variation of tones of stain and patina that adorn my Krags and most of them look great! I've even added grease and heat to parts sometimes to add to the patina and aging appearance. You certainly want it to look good without ever trying to deceive someone to think it really is original as from the factory.

I don't think you need the secret formula used at SA back in the day to get what you want.

When I am restoring or repairing or trying to refinish a part of my Krags, I often start with using a dark cherry stain that I have had for 30 years. I don't have a formula or an exact time I leave it on before wiping it off. I let it dry and use some order of BLO and beeswax based wood conditioner to give it a more even tone and nice clean look. I would challenge most people to look at my Krags and definitively discern which ones have been fooled with and which ones are older finishes.

So, give some simple approaches a try and post pictures. I think you'll find it's easier than you think. It's also fun to try!