John, Trevor, et al., you are correct, the white mounts.
It can be tough to notice the white mount color when viewing them all by themselves above, so well done. Here are some of the other colors from the same source, the Met, most-to-least common.
Standard Yellow (no special color applied to mount)
Standard Pink (no special color applied to mount)
Black
Brown/Chocolate
Red & White would round out the list. The Met has a beautiful red mount of Sam Thompson, it is in the Old Judge book, but the Met hasn't yet posted that scan. There has also long been rumored to be a blue mount cabinet but no one seems to know where it is or have a copy of the image.
I will close by saying that the images I posted at top absolutely look like white mounts, but it really should be looked at in person before re-writing the books.