My guess is that it is kip skin or just plain old high quality cow leather with a nice coating of oil/wax. My understanding is that this was later blancoed by the Army, who then later switched to lampblack for a uniform appearance with less effort.

As for why you don't see it anymore, in mind it's simple: the cheap, shoddy products in circulation today are more cheaply produced in black. One needs high quality leather to look good in lighter colors like natural tan...