['68OrangeSunshine]

I have a '68 C/10 stepside. I bought it in 1973 so I know what is still original on it. It has a spare tire with a crossmember that hangs down on a 1/2"(~) bolt on the drivers side and is held up in place by a another bolt that has a special nut with an eye for a padlock. This mounts on the lower frame just behind the differential. Other early stepsides had an indentation in the drivers side rear fender for mounting the spare there. [Maybe 4x4s?]
Possibly some POs wanted the underframe space to mount a Blazer type fuel tank either to augment the behind-the -seat tank or replace it.
The only evidence of a stock spare installation, if it was deleted, would be some 9/16" [or so] holes in the rear frame between the differential and the rear bumper.
Bolts might be 3/4" diameter. LMC illustrations are correct. I remember getting the "Cross strap retainer" padlock nut from a Chevy dealer back in the day. It had nylon threads -- maybe for vibration mounting -- and the original had disintegrated. I remember grumbling at the price then. I now back it up with a wrap of chain around the frame and secure it with the padlock. That way if the nylock nut turns to powder again, the chain's still got it.
This type of spare carrier was not limited to stepsides. You can check vintage fleetsides, too.