Andrew,
This was cryptic and I am hoping I get close as I take a stab at it.�
Nothing attaches to the side of the frame that sticks out anywhere near 15" that I can think of except the front bumper and the rear crossmember (sometimes referred incorrectly as the rear bumper).� It is 4" wide and does stick out about 15" on the drivers' side.� Each end is tapered but it sticks out further to the left (drivers') side to allow for a license plate to be attached and centered under the tail light.
It should be held to the frame rails with flat head rivets but could have bolts if it was replaced.� If you have a Draw Bar (trailer hitch) then there should also be an insert between the frame rails called a Reinforcement.
What has me stumped is your post indicates you have 2 on each side.� The rear crossmember should be continuous all the way across (as is the front bumper).�
See this article for some images:
http://cj3b.info/Tech/RearBumper.htmlAlso:
http://cj3b.info/Dating/DatingFrames.htmlI'm referencing #6 in this illustration (#1 is the front bumper).
Another image where you can see the only thing sticking out is the rear crossmember (front bumper is missing in this drawing):
http://www.cj3apage.com/index/Tech_Tips/Tech_Drawings/frame%20dims.bmpI could be wrong and off on a tangent.� Could you post a picture?