Isn't thatan external chokewith threads on the outside of the barrel?
And some of those models came with an externally attachedPolychoke.
If it's external, then you need to find a factory Mossberg replacement.
If it's aninternal choke, you can send thechoke into this outfit tohave it matched up just in case it's an aftermarket installation.You can also call them for advice:

http://www.carlsonschokes.com/