Bloody hell. I had never heard of this before Mark Evanier pointed it out, but apparently this opening to an episode of Flying Circus was snipped away when it was re-run a few years later and then the footage was never un-snipped. And it's stayed snipped ever since, including on all the DVD releases and re-runs you've seen. David Morgan, the man behind the book Monty Python Speaks, snagged a copy from a Canadian collector--which may be the only copy of the thing in existence. Be sure to check out Mark Evanier's post and Morgan's explanation on the YouTube entry (feedreaders can watch it there as well) to get the full skinnee.