I would say early / mid 70's as I believe they are the call letters (lic. number from FCC) below the CB handle name. I don't know when they went to a 4 letter + number Lic. as I only remember a 3 letter + number Lic.
Then again the box could be older but the call sign later?
I also remember the CB boxes were big back in the day.
The size looks to be about the same as my Radio Shack unit from the 80's
Dave ----