Definitely not a mainstream, commercial extension, at least not one I have seen before.
Most cuemakers need to upgrade their extensions. It's been an afterthought for too long. That said, the truth is demand is limited. Very few of the experienced players at my pool hall use one, for instance.
I do, and I've fiddled around with many brands. All of them are very light and weight really isn't an issue.
The Predator Extensions are pretty good. Quick on and off. But not stackable. They need an update.
Cuetec was probably the first big cuemaker in the U.S. to really up its game with the Duo extension set, a three- and six-inch combo. It goes on very quickly and smoothly. Sometimes I use just the 3-inch piece. I do wish Cuetec added a few different sizes. A max 9-inch extension is 2-3 inches too short imo.
Longoni has perhaps the best overall extension. The Italian company uses metric, so its main stackable set is a 3.9 inch and 7.9 inch combo. Or 11.8 inches combined. I find it to be more most versatile and useful than the Cuetec Duo.
What's more, Longoni also has 2-inch and 10-inch extension. All of these are stackable.
The Longoni extensions go on with a quarter-twist or click. Superfast, but not as smooth as the Duo. A bit stiff.
Lucasi just came out with a set of stackable extensions similar to the Cuetec Duo, but with more options. Easy on and off, but not quite as smooth and secure as the Cuetec and Longoni ones. They also only work on a handful of new Lucasi butts that go with the InFuzed line.
Bull Carbon has a nice set of stackable extensions, but they don't work with most of the sticks the company is now selling (The LD line). You have to use a different Koda extension that is not as good.
Pechauer and Jacoby have pretty good 4- and 8-inch extensions, but they are not stackable.
The new line of extensions by Cuelees are stackable and well priced.
Budget Billiards online makes an assortment of aftermarket extensions that are versatile and well priced.
The biggest letdown? Triple 60's Whyte Carbon shafts are terrific, but the extension for Viking butts is merely a rebranded Tiger extension with a fatally flawed rubber screw-on system. Doesn't go on quickly or easily like the others. Viking has to do a lot better.
Think I even saw Gorst using an aftermarket or custom extension with his Whyte Carbon shaft. That should tell you something about the Viking extension.