Look on eBay for "U.S. supplements similar to White Ace". There are a couple of people offering. Latest word, White Ace sold, but production of pages will take some time according to the old White Ace website.
"There was some recent release (I can't recall at this moment) where there were four designs and each design also came in its own sheet. Visually (from the front) each design from the block was indistinguishable from its solo counterpart, but from the rear you could tell which was which by the backing paper. So I would maintain they are two distinct stamps, but Scott said its only one stamp."
I guess I have to differ (again). I'd call two stamps that were identical from the front -- you know, the part of the stamp we collect -- identical. I don't collect the back of stamps, one reason I've never been enamored of the whole "never hinged" obsession. I'll take good-looking hinged stamps all day every day. And if the backing paper varies, well, it's supposed to be removed under normal use so that means the stamps are, in fact, identical. One exception might be if you display your stamps backwards, showing the backs of the stamps. No one does that, do they?
People do not just collect designs. Some collect production varieties of stamps so many differences may not be seen from front, back, or side. I collect tagging varieties, some collect gum type varieties, etc. The diversity means there will be differring methods but neither is better or worse.