Sham Shui po post office in 1951. A news report in 1956 indicates the post office was moving to Un Chau Street. It also mentioned the one in this picture was in the middle of the Nam Cheong Street (南昌街) where a nullah was located between Un Chau street and Tung Chau street. On google map, you can see there is a whole stretch of sitting-out area nowadays after the nullah was covered up. The report said the location is inconvenient during busy hours as the queue gets long and people spill over into the middle of the traffic.
As for the the exact location, any information is welcome.