The DM Lefever made single barrel trap guns are very rare. I doubt that's what we are talking about. They have very low serial numbers (up to 3 digits).
The Ithaca guns are much more common. They will have a 5 digit serial number. For those guns try here:

http://www.diamondgunsmithing.com/


Great-Great Grandson of D.M. Lefever
www.lefevercollectors.com