Is there supposed to be a picture of them? Dual horns were a somewhat common accessory. Here is a set on a 49' with the correct relay.
http://www.flickr.com/photos/advance-design_parts_co/8130448691/in/set-72157631870734114
A picture of another set off a 48' I believe, they have a Hi-Lo tone and each is marked as such.
http://www.flickr.com/photos/advance-design_parts_co/8129337811/in/set-72157631868452754
A factory photo of the set from a the 1951filmstrip "Good Prospects."
http://www.flickr.com/photos/advance-design_parts_co/8129454392/in/set-72157631868452754
There is detail on how to install these horns in the individual year Chevrolet Accessories Installations Manuals. If your looking for the 53 horn it is just one (Hi note) that installs to compliment the existing installed horn.

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr