MK VI-S: The same as our MK VI safety but with the tension detent moved to the same position as that on a Saiga rifle safety, eliminating the "double friction line" caused when standard AK safeties are used on Saiga rifles.
Per BATFE, safeties do not qualify as a "US part" for purposes of Section 922(r) compliance.
MK VI and MK VI-S Safeties: Selecting the Proper Safety for Your Rifle
The simplest and easiest way to determine which safety is appropriate for your rifle is to LOOK AT THE SAFETY CURRENTLY ON YOUR RIFLE. If the tension detent is at the TIP of your safety, you'll want to use the Mk VI-S Safety. If the tension detent is a short way BACK from the tip, you'll want to use the "standard" Mk VI Safety.