Stevens 320 pump field grade 20 GA choke

Bought this shotgun recently and it comes with a "modified" choke. I'm trying to figure out what choke it needs to shoot deer slugs ? I contacted Savage / Stevens and they said I need an "in cylinder" choke for shooting slugs. Haven't seen a "cylinder choke" or "in cylinder choke" listed any where I have looked. Win Choke was suggested but I haven't seen a Win Choke ""in cylinder" I've been searching the web and specifics seem vague about application to this model. I have come across "rifled chokes" which I'll assume are for slugs but not sure they would fit this barrel. I watched some You Tube videos and some say the modified choke is fine for slugs just less accurate.. Any information will be greatly appreciated.