Allow me to expand on the previous, excellent posting by Montrala.
The N S settings on the left side are for the second generation gas adjustment valves from HK, with front adjustment and requiring no tools, as visible in Montralas pictures.
The first generation Handguards and gas blocks had an allen key adjustable setting on the right side.
Gen 1 AGR that required use or allen wrench was in fact quick fix when out of the blue Norway requested that they can't live without one. HK quickly modified existing gas block to accommodate this. Then they went back to drawing board and after several years came back with proper AGR in HK416A5. Meanwhile Norway decided to get rid of AGR because it introduced more problems that it solved (actually in first place they wanted it, because proper piston rifle must have AGR, not that they had any real reason to use it). Whole story.
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