Well, I've had 3 of the 8,600 GVW trucks, and all of them came with the FF 14bolt, and their was no option code for the FF in the data list.
They were standard equipment, and all were 350s.
Edit; one of them was a GMC, and for some reason it was 8,400 GVW,, but you know, GMCs are supposed to have lock washers on the bolts holding everything together.
But, Chevy/GMC have done lots of things over the years that don't show-up in the literature or codes.
I think that in the past a lot of trucks got shipped out the door with whatever stuff they had at the moment on a Friday afternoon.