I chanced upon this Straits Times notice from January 1977 - the transfer of telephone lines from Paya Lebar Telephone Exchange to newly-built Yio Chu Kang Telephone Exchange, necessitating the changing of 3,000 telephone numbers.
This was back when:
1. Everyone relied on land lines to communicate! Today, our household doesn’t even have one. We just use our mobile numbers.
2. Most of the area on the map was rural, with just 3,000 households with telephones. Now, the area covers parts of Ang Mo Kio, Hougang, and Sengkang towns.
3. Telephone numbers in Singapore had seven numbers.
4. The Telecommunication Authority of Singapore existed. In 1999, it merged with the National Computer Board to form the Infocomm Development Authority; in 2016, the Infocomm Media Development Authority (IMDA) was formed from the merger of the Infocomm Development Authority and the Media Development Authority. IMDA is presently a statutory board under the Ministry of Communications and Information.
Also, the two place names mentioned as the boundaries of the region on the map - Lorong Lentor and Jalan Woodbridge - do not exist anymore in their original forms. Lorong Lentor has been expunged, although the place name “Lentor” has survived in other roads, most notably Lentor Avenue. Jalan Woodbridge has been renamed Gerald Drive.