Apparently Colt did make some King Cobra's with vent rib barrels.
If you want to spend $75.00 Colt will send you an Archive letter about your Colt.
This will list when it was shipped, who it was shipped to (probably a bid gun distributor) in what configuration it left the factory in, and how many of the same models were in the shipment.
I'd assume this would mention the vented barrel but possibly not.
You might get more info from some of the collectors on the Colt forum:
http://www.coltforum.com/forums/