As marcoR said, those four little pixels at the far right of the letters file are used to fill in the center part of the letters file.
If you want the middle to be fully transparent, just delete those four pixels that are in the bottom right corner of the letters file.
I had the same problem too when I was working on the wood letterbox for End of Ages until I found those four little stray pixels and saw how they worked.
Your letterbox looks quite good Tony so keep up the good work.
Take Care & Pixels Beware!,