Fun fact: in portal there is an out of bounds area in chamber 17 that has a momentum puzzle and some unused models from half life 2 that aren't seen anywhere else in portal. this was already discovered, but im pretty sure barely anyone knows about it.
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pretty sure that's just the extension from the advanced version of the chamber
maybe it is. not sure.
yeah you were totally correct, im still sure about the hl2 props being unique to the chamber tho