Glad to be of any help :) And it is coincidental that the record of the Ahmad Khan i took down happened to be your late grandfather. Is your grandmother still around (in Singapore)..? Perhaps she or your aunt knows a little more?
I also have an uncle who used to serve as prison warden. Perhaps he might know someone who knew a little more about the late Ahmad Khan.
The very least is that you have some letters! Can you make out the address of the sender?? This will help alot in trying to locate your family there.. Do you physically have them with you at the moment..?