NOS vintage Persons-Majestic "universal" seat clamp allows you to use your desired seat on a 5/8" OR 7/8" seatpost top.  Will only work with a matching Persons seat so look carefully!  The grooves must be the matching style on your seat so that the grip is solid.  Seat clamp bolt diameter is 7/16" or smaller will work.  Made in the USA.  Photo is of the part but not the exact same one that will ship.