"A" Stamp

Title: "A" Stamp
Description: 1978 One "USOPS Fables" First Day of Issue envelope featuring the "A" Stamp. Postmarked Memphis, TN, May 27, 1978. Signed by Doris Gold. $5.95 from DK Stampman through eBay, July, '18.

Catching up on cataloguing stamps led me to realize that this is a whole series of sustained UPOPS FDC's, and so I went back to see if I could find the missing letters A, B, and D. I had luck so far only on this "A" envelope. "Doris Gold Cachets" here will become simply "Doris Gold" in later FDC's. Was this "A" stamp worth $.15? I believe that no particular "Fairy Tale" is depicted by showing these two children playing with a building/spelling block.
Source: $5.95 from DK Stampman through eBay, July, '18.
Date: 1978