A Dual Printing Error In A £1-40 Booklet.

I was fortunate recently in purchasing an unusual £1-40 Preston Mill booklet, the vendor had advertised it as a booklet with doctor blade flaws (streaks).
Having purchased it I noticed that two of the stamps attached to the selvedge also had a partly missing background.
This is the complete booklet in question. >

Here is the right hand section of the booklet (enlarged). >

Here is the section (enlarged) with the partly missing background. >

I believe I have discovered quite a rare booklet that is possibly unique having a dual printing error, if anyone else possesses a similar one I would like to hear from them, thank you, WM