This is a Ergo Grip Rubber known as the Never Quit Grip, model 4965-BK.
ERGO's Never Quit AR/M4 Magwell Grip reconfigures the squared shape of the AR/M4 magazine well into an ergonomic and comfortable grip option. It gives AR-15/M4 receivers a distinctive aesthetic upgrade.
Features:
Easy to install
Oil & solvent resistant
Must have mil-spec trigger guard for installation
Cannot be mounted to lowers with permanent trigger guard
