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:

QTHADIF DXCC NameADIF DXCC NoDXCC statusDatePrefixComment
SingaporeSingapore381CurrentUntil 1963-09-15VS1
SingaporeWest Malaysia299Current1963-09-16 until 16:29:59z on 1964-08-31VS1Singapore a deleted entity in this period.
SingaporeWest Malaysia299Current16:30:00z 1964-08-31 until 1965-08-089M4
Singapore a deleted entity in this period.

SingaporeSingapore381Current1965-08-09 until 1966-01-319M4
SingaporeSingapore381CurrentFrom 1966-02-019V


Wikipedia - Singapore in Malaysia

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