Metal Gear REX

Ok, I consider myself a fan of MG, but the lore is something I barely understand and what not (seriously, MG lore is more confusing than Kingdom Hearts, change my mind), but did Metal Gear REX always have that laser or was it just in MGS4?

What do you think? Was it salvaged and taken to Shadow Moses? Can REX stand up straight if it had enough room? I know that REX can fit a fully grown man and Sahelanthropus could only fit a child, but it could have been changed during repairs. Leave you're opinion in the comments below.