From my experience there are multiple '67-'72 in bed spare tire carriers:
- 7/16" slot for 6-lug trucks (I have one)
- 1/2" slot for '67-'70 3/4-ton and '71-'72 1/2-ton two wheel drive (I have one)
- 9/16" slot for '71-'72 3/4-ton (I've never seen one)
It appears GM matched the carriage bolt to the truck's lug nuts to allow a lug nut off the truck to be used.
What size is the slot on this one?