that looks like it could be a shorter Dytac outer barrel, which has a larger lip that doesn't need an adapter ring. Otherwise you'd find there would be an adapter probably wedged inside of the barrel nut. BY the looks of it, if you keep the outer barrel, any aftermarket rail should bolt straight on. The gas tube will be missing because the gas tube on that barrel looks much closer to the receiver than the normal m4a1 setup.
I've no specific rail recommendations, but any AEG rail should bolt straight on. Dytac stuff is nice if you're fitting them once and leaving it. I found their aluminium to be really soft and warp slightly when taking on and off repeatedly. Madbull has the most variety, althought Angry gun have just came out with their mk16 rail. I'm sure they do a 9" version