What is this knurled object on the end of the barrel?

[bigoledude]

My nephew just gave my grandson an absolutely perfect Model 1200 pump 20 gauge.  My brother left it and many other firearms to this only-child when he died. 
I didn't have a lot of time to hang around because it was very late for the boy to be up on a school-night.  This little pump-gun has a knurled thing on the end of the barrel.  It appears like some kind of removable choke with a set-screw.  Does anyone have an idea what this is?  I can't imagine this being some after-market addition since the gun appears no have never been fired.   

[gunnut69]

Winchester pioneered the inter changeable choke tubes that asre now pretty much universal. The tubes with a knurled section extending past the end of the barrel were the first versions made. They were called 'winchokes'.. In a small bore they bring a premium and they are very nice shooters..