I've noticed more recently that most "Grant's" Zebras in the US have been hybridized with Burchell's to varying degrees. It does appear, at least physically, that these are probably true Grant's as far as I can tell.
You can tell a pure from a crossbreed from the color of the white on the hind and back legs. If it's anything other than "pure" white and has any amount of brown or yellowish coloration in it, the animal has some degree of Burchell's in it. I've noticed A LOT of zoo Grant's have at least faint degrees of hybridization.
@birdsandbats That I have no idea unfortunately. I do know the majority of the animals I've photographed appear to be impure... San Antonio's looked good from memory, though.