Going through my Great Britain and I came across this. It's mint, never been exposed, the booklet cover has always covered it and I took pictures at different angle to show that there's no transfer from anything that could do this. What is this? I went on the site SOTW as recommended and the picture of regular is a gradation shade. However, mine has a define strip that runs along the left side of every stamp. Can anyone tell me what this is? At certain angles the strip that runs through is very clear.
As people said, these are phosphor bars. The 9p stamps from this booklet came with phosphor bars on each side. When irradiated with short-wave uv-light, they should have a violet afterglow. These are common. This is one of the stamps on which the phosphor bars tend to be quite visible even without using tools.
As for terminology: what you have is a booklet and the stamps with selvedge that are attached to the booklet cover are a 'pane' for Machin (the name of this type of stamp) collectors, whether it is a 'sheet' or 'booklet (pane)' stamp makes a difference.