Amazingly, that's a really good question Peter.
You made me pull out a few and take a look. It turns out that the first 8 have identical backs - because the comic story extends to the back page. After that they are different. Here's the NZ #9 (upper) and Australian #10 (lower):
So, for 1 - 8 it's only the staple (and the tiny indicia on p4) that's the diagnostic feature. The backs of the Australian ones from #9 up vary, and but are always a Mobil ad. Here's two more (#22 and #23):
For completeness, I should mention that half a dozen issues were reprinted in NZ in 1984. They're the same size and format, but they have 8 extra pages of puzzles in the middle and - more importantly - a different Mobil logo on the cover, and a Pan Am (remember them?) ad on the back: