Sometime around 1976, the government standardised the numbering of homes.
Around the British times, a ground floor unit for eg. Is numbered say No. 20.
A unit above this is numbered 20A, the 3rd storey is numbered 20B.
But the folly of this is if it is a high rise exceeding 20Z, what happens?
Before 1976 (thereabouts): a flat address could be
Block B, unit 508.
But after 1976 onwards this flat is known as Blk B, #06-08.
The government added an additional storey: the ground floor unit or for HDB flats with a void deck is added to the count.
This is why 508 changed: where the 5 = 06 and 08 remains as the stack number.
Photos of homes numbered under the colonial days. These homes are at Katong, Chapel Road.