So, we all know as a matter of fact, Left 4 Dead was owned by Turtle Rock Studios and Valve at the same time, however, just like any other games that has been re-branded from mod to game, Turtle Rock studio made a mod for Counter Strike Source that did help them start working on Left 4 dead, it is unknown what happen to the 2005 build of the MOD version before they made it into games, the only actual concept of proof was in fact: the map Dam it (the map taken place in a dam) and it's old concept art photo, unfortunately I can't find any footage for it :C