Disclaimer: This is a sample answer. The answer may vary from student to student, depending upon his/her observation and experience.
❉ The difference between the letters is the address mentioned on them. Also, some letters had stamps on them while, the others didn't.
❉ The letters that were about to be dispatched to their corresponding stations had stamps on them.
❉ No, all stamps are not similar. A few letters that were meant to be sent very far had costlier stamps on them.
❉ Yes, the stamp of the post office was present on the letters.