At last I know the meaning! Why Singapore has number on their first day cancellation stamp. It is not the day from the first day of issue as I initially suspected. It is the branch number.
Wait, but I had bought from all different places across the island, why they all came with number "1"?
Because what I had bought was the covers supplied from Singpost Center which is being numbered as 1.
Yesterday I bought this cover at Clementi West post office. As usual the uncle already had some ready-to-sell first day covers, which I bought and again I asked the uncle to put the metal stamp on the cover.
The uncle remarked: "Why you want the metal stamp, it is ugly you know? I have the first day cancellation stamp as well."
My friend, first to realize, asked the uncle, could it possibly the number is the branch's number? And the answer was yes.
So all these ready to sell covers were initially prepared and stamped at Singpost Center. That is the reason why it all contains "1".
Later after work, we went to IMM post office to do the second experiment. We purhased a blank cover, and using the stamp we purchased in the morning, we did a "local" cancellation.
It can be seen that the stamp layout is quite different with the morning cover since I did pasted it myself. And the cancellation stamp came with number "25" (IMM branch number). The IMM metal stamp is not quite clear since it was the first time I stamped using the metal stamp.
And not to forget that I do have my own custom self-made "fun" cover. It is so personalised that I keep the Singapore post logo from the stamp (all the stamps came from same location in the stamp sheet).
It is only one in the world. Guaranteed!
So it means that the "Old Maps of Singapore" cover that I've purchased indeed a first day cover! And I purchased it at SGD3.5 compared to around SGD10 at ebay.
And the difference in the first day cancellation is quite similar with UK. However in UK it is completely different cancellation stamp design instead of using number.