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I assume the Serial Interface board was to try and replace the one you bought with the LCD display that had the wrong address? I was lucky in that the supplied LCD display (yellow-green) had the correct serial board attached for my printer. The only other things I bought besides the Display was a rotary encoder and the cables that were suggested on that guys website.
Would it help you in any way if I took a picture of my board (probably not) and posted it here for you? It's a shame you cannot get your hands on a working display to prove or disprove the actual printer board is good or bad, I guess its not impossible for them to be bad. I also assume that the small Arduino board is hardwired to select the address and not programmed, Is that correct?
Where about do you live? If it's nearby I can post you my working version for you to test your printer with, but if it's overseas then the postage would cost more than the things worth :)