I believe brown to only exist on subsections of 129 and 229 in the later series. Pretty confident 606 and 649 do not come this way, with 649 being a 'lighter black'.
Black doesn't normally fade to brown on these T cards - for example, Recruit black T59 is one of the most common of all T card combinations. They don't really come in brown though, despite allegations they do. I've had thousands of them, not a single brown. I've spent years scouring T68 black RB backs and can't locate a single brown one that has also been alleged to exist. Black can create a brownish appearance from certain uncommon chemical reactions, but I cannot make black T cards fade to brown via light in my experimenting. If a back is routinely found brown, I feel comfortable behaving as if it was printed a different color; a single or very small number of one offs, particularly if bearing glue damage, are probably altered blacks.
Just my 2 cents, probably worth a penny.
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