Don, it's era dependent. The ARA safety appliance drawing dated 1917, presented in the 1922 CBC, only shows one dome platform. I've always just assumed at some point the requirement was increased to one on each side, but never bothered to track down the date.
I don't know whether the two walks were ever REQUIRED. Ed Kaminski told me that nearly every detail of the TANK (not the underframe) was a buyer choice, though of course they had to choose among permitted features. Certainly before WW II, you can find plenty of photos of tanks with either one or two walks.
