It may be that most of this film footage was shown somewhere, but the documentary is designed to make audiences feel that this footage has never been seen, or that, having been seen, it was deliberately _____________.
Which part are you having the problem?
The portion after but
Here the division part is comma near the blank.So the idea before comma is mixed .Some time seen and sometime unseen.So idea after comma should be “amalgam” or similar type of word.Here “but” division" word is far away from blank.