This is a Ergo Grip Rubber known as the Sure Grip, model 4141-CB.
ERGO's Suregrip 2 SCAR Grip is uniquely designed to allow installation on FNH SCAR lower receivers without additional fitting. The A2 sized inner cavity is compatible with many accessories available on the market. The integrated rear upper extension supports the web of your hand and the over molded SUREGRIP(R) material provides non-slip grip.
Features:
Designed to fit FN SCAR rifles without fitting
Virtually impervious to oils and solvents
