Thanks Bus_Guide and ZYX for the tips.
I found this on PAP's website that shows an indication of the bus
depot's location.
http://www.pap.org.sg/Party_History/Timeline_of_Singapore_and_Party/1950s/1950.shtml
Students were intimidating bus drivers by setting up a convoy of
20 lorries at Alexandra Road Circus
By setting up a convoy at Alexandra Road Circus (Princess
Circus), it indicates that the bus depot should be around that
vicinity. The 1984 street directory (does not show the depot,
though) indicates a large enough blank area opposite Princess House
and next to the Circus.
Therefore, we can conclude the depot was located opposite Princess
House as what The_Bus_Guide has mentioned and ZYX has
concurred.
Another mystery closed. 