Hm, no - actually I don't think there is ONE definitive version. I mean one of them must have been first, and thus the original version, but today there are many versions out there. And the good part is they are all good. The best thing is to find one that you're comfortable with and stick with that. And always make up your mind as to what all the positions are before you lay out the cards, even if you want to try other versions than your usual, otherwise I think it would be inviting confusion to your reading...!
I read the past not as a time frame, but as the background for the current situation. That means that sometimes it is just a couple of weeks back, sometimes a year, sometimes that position describes life all the way back to childhood for that person. The difference between this card and the below-card for me is that the below stands for the root, where the situation is rooted, and the past-card provides information about where this root has been, the past environment that has supported the growing of the situation for whatever timeframe.
I hope this makes sense...! And remember, this is just the way I read the positions, I may be the only one who does it this way...!
Light and love,