AFIK reinforced buffer tube is part of OTB package. I was first an option, then became standard. MR223 had it from start back in 2009. As far as MR556 I suspect that tube used initially could be US made part and initially. Is tube used now fitted with stock play reducing detents?
BTW I managed to put "mil-spec" Magpul CTR (or was it MOE?) stock on my "commercial" HK tube in my old MR223. It was not "quick adjustable" but sill could be adjusted by hand with some force. It was rock solid. Of course detents were completely unscrewed.