Difference in steering box covers?

<font color="blue">Can anybody shed some light on this? Both covers are marked "GPW-3580" on the inside and both are F marked in the same place. Maybe one is "early"?

I was starting to put the OD one on the box and happened to notice the difference in the other. Any ideas on why these are different?<img src="emoticons/icon_smile_shrug.gif" alt="Shrug"></font id="blue">
<img src="http://todd7359.mypicgallery.com/jeep-pics/covers_large.jpg" border="0">

If they are both infact built by Ford, perhaps one design was found to have a weak spot, perhaps when the mold was wearing out on the earlier design a younger person made the new mold and used a different style of adding in the raised strengthening ribs. I would guess that the green one is older. Perhaps these were built by subcontractors for Ford and they met the spec as given by Ford, yet looked different.