Something I suspected but only got confirmation this year is that some of the "Ambersons" stills are actual frame blow-ups from the original film negative, not just publicity stills. Reportedly, Welles had these made himself (possibly to use as visual references in the post-production stage). He allowed Peter Bogdanovich to use a number of them in the "This Is Orson Welles" book and Robert Carringer used others in his book. I'm fairly certain that the stills showing the party guests arguing over the olives, George reading his mother's letter and a number of stills showing Eugene and Fanny in the boarding house are actual frame blow-ups from the lost footage. Some of these photos are in the Lilly Library collection; they are smaller in size (roughly 5" x 9") than the larger format publicity stills and are all stamped "Feb. 9th, 1942" or "Feb. 18th, 1942" on the back. These photos seem to catch the performers "in action" as opposed to the more obvious staged poses of the publicity shots and the ones that correspond to shots that still exist in the released film replicate the camera angle precisely.
Joseph McBride told me personally that Bogdanovich showed him dozens of these "frame blow-ups" around 1970 including one of the film's original final matte shot: Eugene's auto driving away from the boarding house surrounded by tall buildings with dark smoke wafting between them. Unfortunately, that still and many others seem to have disappeared or been misplaced over the past thirty years.