Replacement flicklock operated upper handrest for the black-shafted (2015 onwards) Black Diamond Viper and current model Venom axes - it comes fitted as standard.
The flicklock enables it to be fitted wherever you want on the shaft and can easily be adjusted for tightness.
As far as we can see it probably won't fit many other axes due to to the tapered section profile of the Viper shaft, at least not without a bit of serious botching! But please let us know if you find otherwise!
It also looks as if it would be quite tricky to fit and may damage the paintwork a bit - not that you'd be worried about this!*
*A customer has been in touch to say that he has successfully fitted one to an older style Venom axe, though this did involve a bit of rubber removal.