I have a pair of these cleats/hooks below the tailgate on my '48 Chevy 1/2 ton. It was a Road Department vehicle back in the day. It is believed that these we used to hold a tarp in place.

http://s160.photobucket.com/user/st...novsky/DSCI0587.jpg.html?sort=3&o=68

Check out this set of two holes on the siderails as well where cleats/hooks were likely fastened:

http://s160.photobucket.com/user/st...novsky/DSCI0589.jpg.html?sort=3&o=66

Here is a set of four holes for the cleats/hooks on the siderails:

http://s160.photobucket.com/user/st...novsky/DSCI0588.jpg.html?sort=3&o=67

I would like to find replacements as well. These mysterious holes can drive a guy crazy!