Simpsons Stamps

I have 4 sheets of the Simpsons stamps (4399-4403) with an error in the date the 4 left-most stamps, i.e. the "2" of "2009" is missing. I had contacted Linn's Stamp News to see if this was noteworthy, but they responded that they had heard about the issue and call it, if my memory serves me correctly, a "repetitive freak". It is unlikely that this occurred in a single printing run as I bought the 4 sheets in 3 different post offices in two different states, and 3 of them have the Homer backing while 1 has the Lisa backing.

I'm trying to figure out how common this is. Has anyone else noticed this?

Could you post a pic to illustrate Homer backing and/or Lisa backing please?

Here are the Homer and Lisa backs to the sheets.

Thank you now I see what you mean.

There are four different backs/covers on the Simpsons convertible booklet. Homer, Lisa, Bart, and Marge and Maggie together.