Probably added by a PO. Check your firewall if it's been altered. Check this link and it shows the difference in AC VS non-AC firewalls. If yours originally didn't have AC then the firewall would have had to be cut. Another place you could check is the side AC outlets. If it's factory, then the edge of the holes would be die cut. If it was installed afterwards then the edges would most likely be rough and not symmetrical.
http://67-72chevytrucks.com/vboard/s...d.php?t=471215