You have to find out which city the cancellation mark used in the five stamps is from.
It reads: HARB
This is the low-spoiler guide for "The city of the five stamps" puzzle in Chinatown Detective Agency. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.