ImperialFighter said:

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so although the 'scaling' issues may have to slide, I reckon things would have tied better together if we could at least have seen the brief hint of an 'opposing leg-joint' coming in-and-out-and-in-again added during the approximate 5 seconds of 'prop' footage where Luke swipes upwards with his lightsaber....since the shot seems to show the AT-AT 'lurching forward' in motion slightly, and has matching sound effects already in place....

Great post Imp :) I like the idea…
If I get you well you're postulating that the two joints on this picture are moving nearer and further to one another as the ATAT walks, right?

Unfortunately, as I understand the working of an ATAT, it's not the case…
I think these two joints only rotate on themselves and stays exactly at the place they are in this picture, plus you never see those joints doing the movement you suggest in wider shots. So the forward joint should always be visible in the prop shots with Luke, just like Angel's mockup…

PS: BTW I've posted some ATAT discrepancy stuff on my thread:
http://originaltrilogy.com/forum/topic.cfm/Discussing-about-scales-in-star-wars/post/387107/#TopicPost387107