I made a post back in October 2009 about this cathedral which apparently was inaugurated in 1955. It was torn down in 1975, and its bells were thrown into the lake behind. They were later recovered and can be seen on the steps of the National Museum. I was working there recently and took some close-ups of the decorations on them. They are all quite common Christian themes such as crucifixes and the bleeding heart but unusual in Phnom Penh. There are two bells that are in good condition, and one of which only the top part remains.
This aerial image shows the cathedral in front of the lake, which has now also disappeared