I just don't know the correct answer to your question why Heuer used 16K goldplating.

The first and "probably" the correct answer is that 16K goldplatig lasts longer than 18K goldplating, just because it's harder. The higher the karat of gold...the faster it will wear because gold is not very hard and wears out easily. Red/Pink gold is much harder in compairison with yellow gold too...but in that time-period I think they only used yellow gold for goldplating.

Cheers,
Abel.
: Hey Arno.

: I just looked very closely at my Carrera 110.515 CHN
: (gold-plated version of the covered lug case) and that has
: "16K GOLD ELECTROPLATED EPSA" between the bottom lugs
: too.

: So maybe this is the standard that Heuer used for the gold-plated
: watches? Abel, is that something that is fairly normal, 16K used
: for electroplating?

: Anyone else have gold-plated Heuers they can check for the same
: sort of text? Looks to be between the bottom lugs on both Arno's
: and my watches, but it's very small and faint on the Carrera so
: you might need to look closely!

: Mark