Well.. I thought that the ThreeA REX was based off of Shinkawa's designs rather than Ashley Wood's artistic interpretation of REX. All of the lights weren't originally seen in MGS 1 REX (I think), but they were visible in MGS 4 REX (some of the lights might not have been functioning because REX was practically destroyed by Snake in MGS 1
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).
In any case, having light-up features on RAY does make sense, since it's an aquatic Metal Gear. And I agree, a light-up spine (along with a light up tail) would look absolutely beautiful
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. Not accurate, but beautiful, nonetheless.