Canadian - Oshawa - trucks appear to have been the place where unused SM420s ended up. I have a 1968 that has one, as well as a confusing propeller driveshaft brake that seems to have been a leftover from the 60-66 trucks.
I read this with interest, though, because it explains why my boot looks the way that it does, and it would in fact be original, I guess. I am looking for one if another comes available.
http://67-72chevytrucks.com/vboard/s...d.php?t=795405