From the 1900s - two 1:500,000 maps covering the whole country - interesting borders with Thailand at that time... also, Stung Treng province is placed to the southeast of Tonle Sap lake...
"Dresser par le Capitaine Lunet de Lajonquiere de l'Infanterie coloniale"- no specific date shown.
Download larger copies of the Lajonquiere maps here.
From 1975, an Ecole Francais d'Extreme-Orient map by Jean Boulbet.
This map shows the landscape of Phnom Kulen and sites of archaeological interest there.
Download a much larger version of the Boulbet map here.
Here's a 1905, 1:8,000,000 map of French Indochina from a German mapmaker...
download a higher resolution copy here