After rethinking it, I think the most practical fingering is the thumb on b, and to just have the extra thumb under at the end. I'm still perplexed by the missing a-flat. @birba: That sounds like something that might happen in certain circumstances, but we're talking about Weissenberg in his prime here. I don't think he needed to do anything like that, especially in such a straght forward passage as this. Of course, I could be wrong...