Gameplay footage from the tech demo levels of Half-Life Alpha v 0.52 dated September 4, 1997, over one year prior to the finished game's release.

Full Valve developer playthrough:    • Half-Life Alpha v 0.52 - Full Develop...  
Alpha deathmatch footage:    • Half-Life Alpha v 0.52 - Deathmatch G...  
Alpha co-op footage:    • Half-Life Alpha v 0.52 - Co-op Gamepl...  

Alpha disc posted by the original owner on Reddit:   / update_finally_got_a_copy_of_that_half_lif...