positions 14-16 that do not have 800, 866, 877 or 888 also need to be copied without modification to the output file.

That's exactly what my code should do. If it's not, show us your code with the new condition added, the input files* you used and the output that should be generated from those input files. I'm tired of guessing.

* — Use input files only big enough to demonstrate the problem.