All factory literature announces the new transmission (SM-420) in 1948. This includes the small mailers and the large full-line brochures. So until someone shows me some proof otherwise?
This is the 48 full-line which devotes significant space to the new transmission and relocated parking brake features.
http://www.flickr.com/photos/advance-design_parts_co/8131119337/in/set-72157631872034068
And rebuild slides for the "new" transmission.
http://www.flickr.com/photos/advance-design_parts_co/sets/72157632620943718/