I noticed almost immediately when I received my set of Walsall’s recent booklets with the Royal Mail red stamps that there is a significant difference in the iridescent overprints. The overprint is very light in the book of 12 standard stamps and the book of four stamps for large letters. It is difficult, though not impossible, to see the codes 12 and T or F.
Conversely, the overprint is heavier on the booklet of six, and the codes are very easy to see. In the image above, the stamp from the book of six is on top and the one from the book of 12 is below. The difference is apparent.
The angle of reflection makes all the difference, so here is another image with the stamps reversed and at a different angle. The codes on the stamp from the book of 12 at top are a little easier to see, but they are still much fainter than the codes from the book of six.
I also noticed that the Queen’s head is much darker in the book of six compared to the books of four and 12. I think you can see this in the image below. The stamps from the book of six are the lower pair.
To my eyes there is a noticeable shade difference between all three books. The stamps in the book of four are a deep red, the stamps in the book of six are lighter and have a bit of yellow in them, and the stamps in the book of 12 are even lighter with more yellow.
And, finally, the gaps in the slits in the book of 12 are much smaller than the gaps in the other two books. You can see that in the image immediately above.
I only have one of each booklet, so it may be that the differences I’ve found are not constant. I would appreciate hearing from others who have seen more booklets and what you have found.