That reads plausibly and makes sense. The question is... was it really like that or is that just an assumption? It should be possible to check this using really old jackets. Perhaps there were reasons for a brown core. E.g. a large batch of brown for brown jackets and then dye a few so that you can also offer black from the same batch? Also just speculation.I suspect the "tea core" effect on older jacket is the result of bone yellow or grey core from hasten drum dyeing like Canuck said, and then with exposure, than faded part become brown from oil exhaust or dirt and grime and oxidation and UV tanning or combination of all those, who dyed leather tan and then overdye it with black in the old days??, people in the old days don't like faded leather, they just come to accept it while waiting until they can afford to buy new jacket, the same with jeans, nobody like dirty faded jeans in old days, people just come to accept it as workwear and replace it from time to time, only since the 80's people started to get creative with bleach and pre faded denim, and only recently "tea core" is a thing to pursue in leather as the equivalent of sick fades on leather like it's on raw denim.