Shan-wow! TWO kitties with that unique color
Ours is about 12-14 weeks old right now. I hope whoever adopts her updates us with pictures to see if her color stays. I told her foster Mom to be prepared for lots of applications once she hits petfinder!
My Little Hersh (see below) was a little darker brown than that color at two months (see my avatar) and four months and is exactly the same at two years. And definitely not black, but darker chocolate brown.
I don't think this is from the Burmese allele (cb) at the Albino locus; it looks like a self chocolate / brown , which is from the brown (b) allele at the Black locus.
This color is not common in random bred domestic cats since this allele itself is not very common in the random-bred population ,and it is recessive so it takes 2 copies, one from each parent, in order for it to show. But it is around; I've seen photos of self brown cats at various rescues. And in the Havana Brown outcross program, they've outcrossed to black domestics and got some brown kittens in the first generation! ( showing that that black domestic shorthair carried brown)
Havana Brown mix seems very unlikely since that's one of the most rare breeds in the world. Siamese mix would be less unlikely, that's where the chocolate gene came from and Havana Browns got it from Siamese ancestors ( and those Siamese had a similar head shape , but not as extreme a version of that muzzle as HBs were later bred for.
But it can also be cats who don't have any recent pedigreed ancestors, and just got this passed down on both sides through many generations of domestics.