Can't say I have ever heard or seen the plugs you mentioned. But, I have seen cleaning kits and thread protector holders that are made by HK that fit inside the pistol grip of the ambidextrous lowers. Gordon Miller and Lee Novak come to mind as people who would be able to find one for you.
A forum dedicated to and laser-focused on Heckler & Koch firearms, Heckler & Koch accessories and the owners and enthusiasts that love them! Come join the discussion regarding HK pistols, long arms, NFA arms, HK accessories, HK history and trivia and more.