3100 was hijack code in 1971...
https://www.nashvillescene.com/news/a-nashville-hijacking-38-years-ago-set-the-standard-on-how-not-to-handle-hostage/article_e394a22d-e99d-52a5-b91e-b0ab20d5db31.html
"Ground control calls Downs and advises him to broadcast the recognized hijack code. "Commander 58 November, squawk 3100."
Each broadcast sounds through tinny speakers in the cabin. Giffe listens in, but he doesn't know the hijack code.
"OK, squawking 3100," Downs says."
Norjak was also asked to squawk 3100,, the hijacking code.