March 1962
"The eastern border barriers at the sector and zone border were extended further by the construction of new walls and barbed-wire fences, putting up chevaux de frise, the installation of tripwires with explosive devices and the excavation of trenches, as well as repair work on collapsed walls, damaged barbed-wire barricades etc.," the West Berlin police write in their monthly report about developments at the sector and zone border in March.
Progress report by the West Berlin police for the month of March 1962 (in German)