What did Thelma Lou do for a living on the Andy Griffith Show?

From the site:
Q. What was Thelma Lou's occupation, last name, and where did she live?
A. There are several things about Thelma Lou that are a mystery to this day. The only thing we know about what she did for a living is that she worked in an office in town. Her phone number a work was 596. We also never were told her last name. We don't know exactly what Thel's address was but we do know that her house number was 830 and that the phone number there was 247. She had one brother and two cousins that we know of. (Karen Moore and Mary Grace Gossage)
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