That must be another detail difference between how the USA assembled them vs how GMCanada assembled them
I have found a couple welding differences in the cab assembly
Not welding the bed seemed silly to me all these years because farmers would have to weld on chunks of scrap metal to hold their boxes together at the back end
My solution was to use grade 8 bolts between the sill and walls and make sure they were ridiculously tight

If someone can post the location and photos of the weld for the back of the box I would appreciate it
I might have to weld mine in the future and not feel as bad about it being “not factory”
Thanks
-s