During your third time playing the game, different music plays over the end credits. This music is the "Metal Gear Solid Main Theme" that was primarily featured in trailers, most notably the one at the 1997 Electronic Entertainment Expo.
At the end of the credits (for both endings) there is a short conversation between Ocelot and the President of the United States. In the Meryl ending, we find out that he's Solidus Snake, the third clone of Big Boss.
During your third time playing the game, different music plays over the end credits. This music is the "Metal Gear Solid Main Theme" that was primarily featured in trailers, most notably the one at the 1997 Electronic Entertainment Expo.
In the end credits, we find out that the sick guard who gets his clothes stolen by Meryl (and who later guards Snake in the torture room cell) is named "Johnny Sasaki," the nickname of Hideki Sasaki.