It was this:
Hearts of Age
A brief, bizarre short film he made with Virginia Nicholson when he was 18 or so. Here, it's presented with commentary by critic Jeffrey Lyons.
I first saw this about six years ago at Cinematheque in Toronto, where they were having a Welles retrospective. At the end of this, the host of the retrospective quipped, "Yes, and his next film was Citizen Kane."