Singapore's 6 digit postal code has some interesting history and facts. For the uninitiated, a postal code is basically a series of digits or letters that are appended to the address to aid in the sorting out of mail. In Singapore, the current postal code used is a 6 digit postal code (i.e. all numbers). I still remember the days when it used to be a 4 digit code which was so much easier to remember. In actual fact, a quick check on Singapore's postal code history reveals that there were really 3 different postal codes used.
Postal Code formats throughout Singapore's history
1950 - 2 digit postal code
1979 - 4 digit postal code
1995 - 6 digit postal code
To find out the 6 digit postal code of any address in Singapore, you can use Singpost's service here. All you have to do is provide the block number and street name.
Well, Singapore in the 1950s was divided into 28 postal districts. These 28 postal districts are actually the same as the districts that you will find even today in classified ads for properties. This goes to show how long entrenched the district system really is and it shows how the way postal districts were organised have gone on to influence the way a property is classified today. For example, the district 9 properties (Orchard Road, Tanglin, Cairnhill) are often the ones that fetch a high price in the property market.
Subsequently, in 1979, the 28 postal districts were further subdivided into sectors, bringing to a total of 81 sectors. So in the 4 digit postal codes, the first 2 digits still represented the initial postal district (i.e. 1 to 28) and the last 2 digits represented the sector (i.e. 1 to 81). I guess this method helped in the sorting of mail tremendously since certain postal districts tend to be large. In fact, there was a stamp issued in Singapore in 1979 to commemorate the introduction of this new 4 digit postal code.
In 1995, the 6 digit code was introduced. The first 2 digits were now the postal sector code (i.e. 1 to 81). The remaining 4 digits indicated the delivery point. For HDB blocks, the last 3 numbers refer to the block number (e.g. XXX355 for block 355 and XXX089 for block 89). If there are 2 blocks with the same number in that certain sector, that is where the 3rd digit comes into play. (E.g. XX1089 and XX2089).
Interestingly, another Singapore stamp was issued in 1995 when the 6 digit postal code was introduced. In that stamp, the 4 digit postal code shows 1543 meaning that it was in district 15 and sector 43. That places it in the Katong/Joo Chiat/Amber Road area. in the stamp, the new 6 digit code for 1543 was translated to 430010 where 43 was the postal sector. 010 is thus the block number or house number.
So it seems that there is some duplication in the way we write addresses on our envelopes in Singapore. Since the 6 digit postal code can be derived from the block number and street name, I guess your mail will still get to the intended recipient even if you leave it out(?). Or you could just state the unit number with the postal code without the street name and your mail should reach the correct address. [Warning: This is just my hunch. Try at your own risk!]