When I bought and parted out Camaro's in the 80's to help fund raising my children, I came across a few with trailer hitches.
And when I bought my 67 hardtop from the original owner in California, it still had a trailer hitch on it. They all had the same style of mounting whereby it had two thick bars extend under the gas tank and bolted to the trunk floor and also the bottom of the rear valance and made me wonder if at that time if it was a dealer item, but I doubt it.
Mike