Hmm well I dont know what kind it is either lol. It doesnt have any markings on it. Heres a pic of it next to a coke can for size reference. The gauges go up to 3500 psi and 140psi
That's a regulator. Judging by the nondescript gauges, it looks like a Milwaukee. A diffusor is something that either sits inside the tank ot connects up to a canister return line to release fine bubbles into the tank.