All I can tell you is that in Pan Am timetables for 1950, 1958, 1963 and 1973 there are no cargo services shown to Singapore, and there were not many to the region. Pan Am was always quite precise in showing cargo flight schedules in their main timetables.
All through the prop era Singapore seems to have been operated as a low-frequency appendage, changing aircraft at Hong Kng from the main transpacific type. There was just one DC4 a week in 1950, and 2 DC6Bs a week in 1958. By 1963 there were 3 707s a week coming down through Guam, Manila and Saigon, one of which went on to Jakarta, and jet cargo flights had come to the region but these were not scheduled west of Saigon.
By 1973 Pan Am had given up Singapore completely, despite still serving Bangkok, Saigon and Jakarta in the area.