Steve...took a look...on the same page on the top row, 5th and 6th pic, is the same machine with a little better angle to see that...it does look a bit more handwritten than not and it's on all three wings as well... and on both ends. I don't see any such notation on the wings of any other DrI shown, early or late. But, of course, that settles nothing. Minor mystery, I guess.