Hello. You have downloaded Half-Life 2: Lost Coast: Project-Beta (also known as HL2:LC:PB).
Put this folder into C:\Program Files (x86)\Steam\steam\steamapps\sourcemods\ or whatever your steam folder is.
Close and open steam if steam is running. Then if you did that you will find Half-Life 2: Lost Coast: Project-Beta in your library.
Q&A:
"What is the 2004 era mockup based on?"
- According to Valve's Robin Walker, one of the early versions of the Lost Coast had the Fisherman picking the player up in a boat and bringing them to the Monastery.
- One of the early iterations of Lost Coast (d2_LostCoast_scene.vmf, dated February 2005) contains a very highly detailed path on the other side of the level from where the player actually goes in that version. The original spawnpoint in the map is also on a dock at the start of that path, all seemingly leftovers from the boat ride version. All of this leftover geometry was left unchanged for the mockup.