If it's choked M/F and the length of the choke taper is more than 2 1/2" to 3", it's very likely to be original. MD is correct however; the only way to know for sure is with a Callahan letter:
Letters are $30 for Sterlys and $35 for graded guns.
J.T.Callahan
53 Old Quarry Road
Westfield, MA 01085