on my 92 f150 4x4 i have solid radius arms, i bent one and went to the local junk yard that has the most fords to get one, all they have are ones with a big square-ish style cut out and ones with about 1 inch holes drilled in it. out of 8 trucks not one had a soild arm???

 

Mine's an '87 with the holy radius arms. I don't know if the styles changed, but either the trucks you were looking at were older or a previous owner upgraded the radius arms on your truck.

 

I am not sure how true this is but I was told that the holy arms are heavier and stronger than the solid arms. I believe that this is because they have thicker sides.