Bought from ebay, the GoPro extension arm mount can be use in Hero2 and Hero3 camera. It can coexist with other mounts where it fits. Can’t wait to test it ^_^
eBay item: Blue TMC CNC Aluminum Alloy Extension Arms Mount Screw for Gopro HD Hero2 Hero3, 32cm in total length, aluminum alloy with (looks like) anti-rust coating, and three blue mount screw.