Originally these extra perforations were introduced by the the printers as a reference position to aid the location of bent or missing pins. If you would like to see more of these just type Extra Extension Hole into your eBay search.
On certain values the EEH can be found in one of several positions. They are found in the left margin of the blocks. Copies can be found either in row 18, 19, 20 or 21. I might add, they are all listed in specialised catalogues and are highly collectable.