Not sure about the textures, probably some of the textures that were used during the Hyper Era like some leftovers from the oldest version of the Borealis maps and Odells old facemap. As for models I think that the oldest model that was made for HL2 that we have is probably the old physgun model that leaked recently since it's believed to have been from around the year 2000 when physics were added to the game.