Please let me bump an old thread. I have read about this Madrid expo issue in several threads on different forums, but I do no understand how to positively identify the originals. Hope someone can help?
I had quite a pile of them to work on, so I have sorted them according to what I understand is the best clue: any stamps that are not lineperf 11,5 are reprints or forgeries. Here's a selection of them, all perf 11 to 11 1/4 :

I ended up with several stamps remaining with line perf 11,5, possibly being authentic from what I understand. However, I think I am missing something, as I doubt that all of these stamps are authentic:


There is also something about the color shade, the originals are richer in color than the forgeries. Additionally I have observed
significant size and length/size ratio differences between them.
But which is which?
Some of my specimens are cancelled with what seems to be a 1907 double-ring postmark. Anybody knows if this is an authentic cancel used at the expo?

Any other indicators? Any help much appreciated

Jon