[modeller_usage] Appending model pdb files

As I want to append to one another model pdb files (for chains A, B, C, etc) to build a multimer, may I ask about what I was unable to trace:

(1) What does "1SG" at columns 74-76 (PDB numbering) mean?

(2) Why serial numbers (Atom number + 1) are reported at column 80 (PDB numbering)?

(3) Should serial numbers said at (2) above be updated (like the Atom number at column 11) when concatenating the model pdb files?

(4) Why the Chain identifier is not printed in model pdb files (which is obviously needed for concatenation), or there was a way to specify it be printed?

(5) When programming Python I take for obvious that it starts reading the columns from zero, not 1. OK?

Thanks francesco pietra