I believe that's the menu you should get after you beat the game for the first time.
I know I've seen it ages ago, but cant recall if I got it.
Since Ive deleted and reinstalled portal several times now(don't think I've even beaten it on the PC now that I think of it(Although I've beaten it several times between the PS3 and 360 versions)) all I get is the first menu which shows where Chell wakes up.
Ah, here we go, doesn't explain how to get the menu, but it shows it:
Video from 2007, so it's definitely been there. I do know the end cinematic is different on the PC after a recent patch though.
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