I'm not a specific "70's buff"...but I have asked some of the same questions myself. My understanding from some friends who have "Internationals" is this ~
"Overlander" describes the length/model of your trailer.
"Land Yacht" is the affectionate term given to all(?) Airstreams in the 60's & 70's.
"International" denoted the upgrades package including things to make caravanning & longer trips more comfortable.
I'm sure someone else will pipe in & clarify it for both of us...
Shari