Thanks loads, that is exactly what the piece looks like. So, I can buy diferrent screw in pieces to modify the choke?
Yep, they are generally proprietary to the gun manufacturer, so either buy the manufacturer's brand or go with compatible aftermarket ones.
I've had good luck with Trulock chokes for my shotgunning.
Sorry for a late reply, I have a Remington 870 Express and yes you buy different tubes to create either wider or narrower shot patterns, please educate yourself on that! It would ruin your day if you fired a slug and you had a real tight choke!