First, welcome to the forum.
Actually, the springs you are trying to sell are the standard assembly line springs supplied with 64 thru 66 Mustang Convertibles and 65 and 66 Mustangs Fastbacks with the 289CID HP engine, and yes, they were on Shelbys too. Now don't be disappointed, they are rare and valuable. You just have a wider market.
Jim
The date code translates as follows:
16 = day of year, in this case, the spring were made in January on the 16th, so usable on a car at least a week after (more likely two weeks).
6E = year, 1966; and manufacturing plant designed as "E" (possibly Eaton)