@DOC Welcome to the forum!
Hmm. I wonder how that could happen. It might be that one of them could be a replacement for an error on 777886 or 777885, which was set with the wrong number, in error.
Here's a serious rarity, an error on a replacement note
A printed fold, causing the date to appear on the back of the note when unfolded. I wonder if this replacement was itself replaced.
Lavery Ten Pounds 1975, replacement note error.