More than likely a Tru-choke system installation. Colonial calls their equivalent TrueSize. Hastings also used to make "Tru-choke" pattern chokes. Both chokes below are Hastings.

The only other aftermarket choke installation I am aware of is Briley's system. Briley thinwall chokes have square threads on the muzzle end of the choke, and are generally longer than RemChokes.

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