Did you add an additional 0? There were no Hunter Arms guns with 400,000 SNs. The Field grades started at about 216XXX

A rough guide to DOM is here
https://lcsca.clubexpress.com/content.aspx?page_id=22&club_id=43784&module_id=246742

An exact DOM can be documented with a Research Letter
https://lcsca.clubexpress.com/content.aspx?page_id=22&club_id=43784&module_id=245742

It would be a good idea to carefully review the FAQ "What should I consider before buying a vintage L.C. Smith shotgun?"
https://lcsca.clubexpress.com/content.aspx?page_id=274&club_id=43784

I hope the purchase works out Derek.