Assuming you lived in the same area back then, there are no "brown rat snakes"- but as I said, if it wasn't patterned (see Great Plains-Emory's rat snake), I've seen at least one refer to a black rat snake as "brown"- maybe because we don't all see colors the same way, or because it had red undertones that in sunlight made it look more brown than black? It's like I have 2 black dogs, but in bright sunlight you can see that one has tan hairs mixed in her coat, making her appear to be a very dark brown color, especially when she stands next to the other dog who is pure black. Your rat snake still seems likely to be a "black" rat snake, IMO.