This is a centre-loaded aerial, around 3 feet long, supplied with a medium duty shock spring. It has a solid lower rod, hard rubber encased centre coil (rigid) and whip top section - a good aerial for "heavy-going" off roading, although the only flex is in the spring at the bottom, and the top whip section.
The centre loading coil allows fitment where if at the bottom, the coil would have too much metalwork around it... the fact that the coil is higher can allow it to "clear" the metalwork and minimise any SWR problems that might arise.
The Flexi AT-12 has the same 3/8" fitting as the Springer so will simply screw into any of the standard mounts just the same.
Normal price £12, so to have this aerial instead of the Springer in a package is an additional £2.
You can see more information about the Flexi AT-12 on it's main product page : HERE