I won't disagree with you on that, most likely it is the 60. But I have seen a few single wheel trucks with the 70, all of them were leaf spring trucks with either the 1 ton package/Longhorn C20, or they were Camper Specials/Camper Customs. One of them was a '69 Camper Special that belonged to my Grandfather (my uncle still has it), and there was also a '71 GMC Camper Custom I almost bought. Both definetly had 70s in them.
