I just found a (U.S.A.) Scott #900 2˘ 90mm. Anti-Aircraft gun (10/16/'40)- that appears to have been over-inked and has smears through it. Look directly over the 90mm. Anti-Aircraft gun's barrel and breach and you will notice a few streaks. I know this doesn't add any sort of premium or anything but philatelic-ally speaking it's a pretty neat find for such a common stamp I think anyway!
