I think the reason they say that is because the original "Triplex" was the 4700. The 6700 was an upgrade - called the "Triplex Super", so that was the improvement they're talking about. It's the same basic design, but they removed the complicated central button design that triggered the spring that raised the flint compartment, and instead made the flint cover hinge upwards from the main cap hinge point. They also had a simpler version, the 6600 "Triplex Junior", without the flame adjustment.
Here's a
website that shows the development of the IMCO lighters.
In my view, the IMCO lighters are the best petrol lighters ever made. They outperform Zippos, Ronsons, and every other petroleum lighter I've ever seen - and they're cheap.