She appeared in the 1986 reunion made-for-television movie, Return to Mayberry.
Thelma Lou's debut occurs in the first-season episode, "Cyrano Andy".
Unfortunately, Lydia's extremely odd personality doomed an effort by Barney and "Thel" to arrange a date and possible romantic match with Andy.
The stability of Barney and Thelma Lou's relationship is questionable.
If the photo had been sent out by Rogers and Cowan in the 1990s, it would have a zip code, the phone would be shown as all numbers and there would be a fax number in addition to the regular telephone lines.
en: Don Knotts played the character regularly from 1960 to 1965.
Thelma Lou lives in the community of Mayberry, North Carolina.
Despite their ups and downs, Thelma Lou stated that Barney was the man she wanted to marry and be the father of her children, and Barney once told Thelma Lou, "You're the only one I ever gave a hoot for." He also tells her “You’re the cat’s” (as in “cat’s meow”).
A running gag on the show involved Barney and Thelma Lou spending a "quiet evening together" and somebody (usually Andy) walking in on them to find Barney's hair mussed and lipstick prints all over his face.
Knotts periodically guested on the program as Barney Fife, but after 1965, he was no longer in a Mayberry uniform, etc. The later date stamps appear to have been use dates by the newspaper who had the photo on file.2 The year of first publication.
If the work is a derivative work or a compilation incorporating previously published material, the year date of first publication of the derivative work or compilation is sufficient.
Barney and Thelma Lou are platonic friends but their relationship becomes (and remains) a romantic one with Andy's help.