In the post-WWII period, until May, 1963, Singapore was a separate DXCC entity, with the prefix VS1.

Singapore merged with the Federation of Malaya, Sabah and Sarawak, to form Malaysia, on 1963-09-16.
This change to the political status of Singapore meant that Singapore no longer counted as a separate DXCC entity, but was part of Malaysia.
Contacts with Singapore at that time count for the DXCC entity of West Malaysia. The prefix for Singapore stations changed from VS1 to 9M4 at 16:30z on 1964-08-31.

For political reasons, Singapore was expelled from the Federation of Malaysia on 1965-08-08, and became an independent state on 1965-08-09.

The original DXCC entity of Singapore was restored to the DXCC list. The 9M4 prefix continued to be used until 1966-01-31. The 9V prefix was allocated to Singapore with effect from 1966-02-01.

The following table summarises these changes:

QTH ADIF DXCC Name ADIF DXCC No DXCC status Date Prefix
Singapore Singapore 381 Current Until 1963-09-15 VS1
Singapore West Malaysia 299 Current 1963-09-16 until 16:29:59z on 1964-08-31 VS1
Singapore West Malaysia 299 Current 16:30:00z 1964-08-31 until 1965-08-08 9M4
Singapore Singapore 381 Current 1965-08-09 until 1966-01-31 9M4
Singapore Singapore 381 Current From 1966-02-01 9V


Wikipedia - Singapore in Malaysia

ARRL DXCC List - note #10
DX News Sheet #130, dated 1964-09-08, published by Geoff Watts