Went thru about 3 of the 'low' profile locking gas cap and although I prefer them found they only lasted me a few months before the locking mechanism fails and removing the cap became a problem. Don't recall who the manufacturer was and either purchased from local auto parts store and/or E-bay.

New Locking Gas Cap Ford Truck 51 64 65 66 67 68 69 70 | eBay

Finally went to Napa store and purchased a locking gas cap, gave them the yr. and model and seem to recall there were a couple of choices so I tried the choices before purchasing. Difficult to make out the number it looks like Napa P/N: 7031113, and been on the truck for the past several years. .

http://partimages.genpt.com/partimages/232871.jpg