I thought I'd bring this post up again for new members to see. Maybe there may now be some information in a specialized catalog.
This one has had me puzzled for many years. I don't know what to make of the two C118 stamps on this cover. Is the coat on the right C118 a possible color error of green instead of gray? I have had 100s of this issue and only these three stamps (and maybe one more on cover) have green jackets.
Note: On Oct 26, 2012, I removed a few unhelpful images and replaced them with two new ones displayed below.
There are 3 different variations of the four C118 examples. The top two on the same cover
both have grey shading on the plane's tail but the left stamp has a grey jacket on Sam and the right stamp has a green jacket on him. The bottom two stamps have green jackets and
both have green shading on the plane's tail. The really odd ball is the top right on cover stamp with a green jacket but with grey shading on the plane's tail. (The bottom two stamps are not part of the cover.)

Below is a detail of the two on cover stamps. The left one with the grey jacket shows what appears to be green dots at the top of the collar. The dark dots on the green jacket are black, not grey. They are also on the grey jacket but are less noticeable.
