I read somewhere (and I'll try to find the source and report back) that that is currently the correct address for the 111th, but that before the ZIP+4 code came into being, PO Box numbers were only three digits long.
That would only allow for 999 possible box numbers. The number is unusual but legit. It appears on at least one of the REAL documents from Bush's records.
I thought it merely rested on the size of the post office of the box in question. If it is a BIG post office, more boxes and higher numbers. I reserve the right to be wrong on this.