Not really NFA but...
Unless you have some real love of shooting 37mm flares, or you just want to up the cool factor with those who don't know any better, I would leave it off the gun. I've never shot one on an AK, but I have from an AR15 and they really aren't worth it IMHO. The ones I've seen and handled are usually made cheaply and don't hold to the weapon very securely. Additionally, they add a significant amount of weight and bulk to the gun. Thus the launcher becomes like a very loose fitting handguard when you shoot the rifle, so you have less control than you would with a regular handguard. That is not to say they are all bad, but unless you're going to get something built to the same standards as a real 40mm military unit, I'd either forget it or just geta separate stand-alone 37mm launcher.