The first map has no date, but may be from around 1925. The second, much more detailed one has a date-1928 and the third is from 1993. I have scaled the maps and sized them so the main features are in the same places, more or less.
In the oldest map, Boueng Keng Kang shows as a huge lake, or wetland, south of where
Sihanouk Boulevard runs today. In the later map, the lake is shown only on the west side of what is now Monivong Boulevard. Saloth Sar lived in this lakeside area in the late 1950s.
The Champ de Courses, or racetrack, is in exactly the same location and alignment as the current structure.
Wat Svay Pape is shown as almost on the river bank. I was aware that most of the Tonle Bassac neighborhood had been reclaimed from the river, but it’s a pretty long walk nowadays to the river from that particular pagoda.
There’s an inlet, which I presume was another drainage canal shown in the area that is now Hun Sen Park, all the way from the river to where Independence Monument now stands. More interesting, is a little lake, adjoining one end of the palace walls. Back in those days it was called Etang Sacre, perhaps it had some ceremonial function. This is where the park with the Vietnam Friendship memorial now stands, and it still has a few ponds and water features which were built around 2008 when the park last got a make-over.
The mosques on Chruoy Changva are shown in exactly the same place as they are now, somehow though the one at Boueng Kak, which was one of the oldest structures in the city at the time, is not marked as a mosque. One map shows Russei Keo church which was largely destroyed in the war. On the opposite bank you can see the Keang Khleang church, a little-known place that still stands today.
The whole west side of Monivong was obviously a lake until recently, and Boueng Kak was originally much more spread out than it later became. The better known canal that went along where the park between 106 and 108 streets turned north just before the point where the railway station now stands. The land drained along there was the later location for the Phnom Penh Cathedral, the French Embassy, and the Phnom Penh Hotel.
One of the most remarkable things when comparing the maps is how the Bassac river has changed course, leaving only a small channel where it used to flow, and carving Koh Pich off from what was part of peninsula that Chba Ampul still stands on.
Edit: From what I've read since posting this, it appears that Koh Pich was formed naturally by deposits in recent decades. It was farmed for some of the year but would be submerged in the wet season. Then more recently it stopped being submerged annually and became more permanent before it was developed.