George,
When Ives added the American Flyer 9" caboose to their line they got a car that was a better match to their 9" freight cars.
As you mention in your post, Ives used both the Lionel and the American Flyer bodies for a longer caboose. Both were numbered 121. Personally I like the Flyer version better.
My Flyer caboose also has a very small area of the "orange peel" paint.
Northwoods Flyer
Greg