That strum is
Down-down-up-up-down-up, so you'd voice the down strums (on Am chord) as a quick 6 note arpeggio of (low to high) E-A-E-A-C-E, and the up strums would be reversed.
Dave...I went to "settings" on the Youtube video and cut the speed in half. I listened closely and discovered the full sound comes by playing an "open chord?" And you said it starts on an E (the lowest string?) and it plays over two octaves?
So, theoretically, if I get a good guitar patch I can sound just like that on The Letter and Stones songs and others?