Hi Miko
Looks like they had a problem with the pins in a small area.
Not uncommon and with so many stamps being printed in China, this will be quite a common variety one would think. It's good that it's joined with other stamps and on cover.
If you check closely with a magnifier, you may be able to see if the pins were completely worn or broken or if only part so. You tell this by checking to see if any indents are showing. This is easier to tell on Mint stamps.
Stamps and sheets are often found with 'missing pin hole/s' and are quite collectable. Your example has quite a few missing. Surprised they didn't notice this problem.....or then again, maybe not !
In Gibbons catalogue they are listed as 'Missing pin hole' varieties.
Londonbus1....Missing many pins
