This challenge was envisioned by bigcall (and named by The Leprechauns) to honor Charter Members. To quote his original:
With over 6 million registered cachers there are less than 500 "Charter Members". Charter Members are those cachers who were the first paid Premium Members back in the 2002 through early 2003 timeframe and who have maintained continuous paying membership since. See the official definition here. A post from the forums by Moun10Bike on 23 January 2014 puts the number at 491, (see: this post), and it may continue to dwindle.
To qualify for this challenge, you must find, or have found, caches that are owned by at least thirteen (13) different current Charter Members. This means that even though the owner may have been a Charter Member at one time, if they're no longer a Charter Member at the time you log your find, their caches are no longer eligible. Caches may be of any type (even locationless), and it's okay if they've since been archived.
To help with this challenge, Moun10Bike has posted a list of active caches owed by charter members here. Fizzymagic has made two GSAK macros: one to check your finds and another to identify unfound caches hidden by charter members.
When you log your find, please provide a means of verification. The two easiest ways will be a bookmark list, or a screenshot. Here's my bookmark list of my qualifying finds.