Version 1.52 is same to 1.51 with some minor bugs fixed which were introduced in 1.51 version.

Version 1.51 was made by asb2 player from version 1.5, it has no drop from the drop ship and dead bodies were converted to biomass. It contains the original uncut Extermination campaign version 1.5. This means that when this client hosts a lobby and somebody joins, that somebody will download these *.NAV files too. But if this addon is installed onto a server it makes all clients who connect to this server to download separate *.NAV files which sometimes take a lot of time and 'infecting' this client.

navmeshes.vpk is not useful for Alien Swarm because AI in this game doesn't use navigation meshes which this addon adds. The primary differences of this map pack to the 1.3 version are these: Maps that are released after the map pack are available in the announcements section. This map pack contains all new maps as of 2014 September. Contains the latest versions of all maps together with new maps.

Alien swarm map pack