Hi, Bill -
The Carter Family copyrights are held by Peer International Corporation. Ralph Peer is the man who recorded the Carter Family and Jimmie Rodgers at Bristol, Tennessee, in 1927.

The Music Publishers Association says that Peer International rights are managed by Hal Leonard:

Hal Leonard Corporation
PO Box 13819
Milwaukee WI 53213
Website:
www.halleonard.com
Clearance Representative:
www.halleonard.com/permissions/index.jsp
Email:
hlcopyright@halleonard.com

I'm an associate editor of the Rise Again Songbook, which will be published by Hal Leonard in August 2015. Our Hal Leonard licensing guy said not to bother trying to disprove Carter copyrights, but I didn't get an explanation why since he was taking care of them. Now I see that Hal Leonard manages the copyrights, so maybe we had no need to bother challenging them.

-Joe-