It fits and works fine, and it's the latest m33 version - MFT M33 Military and Police 2 Sided Rail M-4 Carbine

Despite what the product description reads, this is the latest version of the M33 that doesn't require a separate heat shield. It's a newer version M33 that's made of a higher heat resistant polymer. I've fired a few hundred rounds thru my M4 and it works fine without the shield. It also fits fine, but you should use an "AR Hand Guard Removal Tool" to get the old hand guard out and get this new one in. It looks great and the rails works just fine.