The early G15-N and G15-M models (aka. Atlas Scrambler) featured special dampers and external springs derived from AMC's own "Teledraulic" forks, providing the "Roadholder" with two-way damping.
This version should be compatible with JS Motorsport upper bushes offered for Commando forks. Dampers are still available from Norvil Motorcycles and possibly others. Re. springs, maybe Russell Motors still has them. I hold a set and may provide dimensions if you want to have a pair of springs taylor made. I can't remember if the conversion included buffer springs - please check the '63/'64 parts book. Buffer springs would help to minimize the "bottoming out" impact.
Sometime in 1965/66 a further rationalization of the G15 range took place at AMC, and the "hybrid forks" were dropped on production models, maybe realizing that few of the G/N15CS bikes would be raced and rebound damping was considered sufficient for a street scrambler.
For a quick check of your forks, just raise the gaiters to verify weather external springs are fitted or not.
"Topping out" occurs when the suspension extends fully and cannot mechanically extend any more, producing a metallic clunk. This is a common feature to all Roadholder forks. The effect can be reduced by improving rebound damping and careful adjusting the oil level. Follow Jim's advice on how to achieve best rebound daming, and get rid of corroded damper rods.
For improvement of compression and rebound damping, please read Jim's web page (J.S. Motorsport) and this link:
https://www.nortonownersclub.org/support/technical-support-common/roadholder-forks
Modern damper performance of the late G15 forks may be acieved by using the Cosentino Engineering fork kit (i.e., cartridge dampers) and appropriate damper rods.
-Knut
