In the Brian Sweet book 'Steely Dan Reelin' in the Years' it mentions that Walter did play the solo on the Pretzel Logic track, it took a long time to piece together, often "an hour per bar", according to Roger Nichols.
I've just been listening to an interview with Walter on you tube reference 'Walter Becker The Rare Tracks Collection: Obscurities and Oddities' and at around 57:32 he is asked which songs he is playing guitar solo on and he mentions Pretzel Logic! :-)
In the Brian Sweet book 'Steely Dan Reelin' in the Years' it mentions that Walter did play the solo on the Pretzel Logic track, it took a long time to piece together, often "an hour per bar", according to Roger Nichols.