I wasn't there and there isn't much from sessions, but when Dave is in the rhythm mix, it seems Mick is on a whole other level of playing. He's more aggressive and inventive IMO. Maybe it's because him and Dave didn't really get along OR maybe they were in a good period between 67-70. If you think about Dave's songs coming around this time, they're not a standard 2/4 arrangement. They're pretty wild. "Walter" seems to fit in this category. I have reason to believe Dave had a lot to do with how this song flows.