156 Transparent Deep Blue
Used for the eye in 8009 R2-D2 (2002) and the Star Pilot watch (2000). It's nearly identical to 43 Transparent Blue, but perhaps slightly darker. One theory is that, because the boat plates are rather thin, a darker color than usual was needed to match other Transparent Blue parts.
This color sort of glows faintly under a UV light, especially the links. I'm still suspicious that the 2x2 boat plates were mislabled by Lego.