The term "karat" is primarily a jewelry or decorative (e.g., gold leaf) term and I have NEVER seen or heard it used inside the electronics industry in reference to the gold purity, at least as far as plating is concerned. The purity of gold plating on electronics is always given as a percentage and most all you've ever seen, whether new or old, ranges from 99.7% (hard gold) to 99.99% pure (soft gold), depending on the application. In electronics gold baths, small amounts of Co or Ni are added to increase the hardness of the deposit and, thus, it's wear properties. If more alloying were used, it could severely affect the solderability, electrical conductivity, and other electrical properties.
For jewelry gold plating, you will see the terms, 22K, 18K, 14K, etc., used. However, these most always refer to the COLOR of the deposit and not the purity. For example, most 14K jewelry color gold deposits are actually about 20-21K in purity.
Any two metal gold alloy plating bath containing more than about 1% of an alloying metal is much more difficult to control than a one metal bath. By control, I mean maintaining consistency in the exact alloy deposited. In jewelry plating of color golds, which contain large amounts of alloying metals, Al Weisberg (see below) says that a pH change of only 0.25 units or a temperature change of only 3 degrees F in a plating bath will affect the deposit color and, any change in color is also a change in the gold purity of the deposit. Also, the current density and the amounts of the other constituents in the bath, which are ever-changing, can affect the alloy. Three metal plating baths are virtually impossible to control.
Here's a good paper on gold plating written by Al Weisberg, the founder of Technic, which is probably the world leader in providing gold plating solutions to industry. I think it's about 30 years old but nothing much has changed since then. Al also wrote the article on gold plating in all of the more recent issues of the Metal Finishing Guidebook, which comes out annually and is the Bible for the plating industry. The Guidebook article is nearly identical to the one in the link below. In the older issues, the gold plating Guidebook article was written by Bob Duva, my mentor at Sel-Rex, where we both worked.
http://66.192.79.183/articles/079702.html
The common electronic gold plating solutions used when I worked at Sel-Rex (late '60s, early '70s) were virtually the same as those used today. The main changes today are in some of the equipment used to apply the gold (called selective plating, zone plating, spot plating, etc.), on what parts the gold is applied, and the thickness that is applied on certain non-wear surfaces. For common wear surfaces, such as fingers, you needed the same 30 micro" of hard, nearly pure gold back then, that is needed today.