When the FCC first issued licenses in the late 1950s, the licences were a number, a letter (mostly Q), and four numbers. The leading number referred to a district; they more-or-less followed -- to a large extent -- amateur radio call areas. If the licensee traveled outside his home district the licensee was required to notify the district FCC engineers in which travel/use would occur. The notification process was similar to that required of amateur radio licensees.
My call was 2Q3671 (issued in 1960). The "2" represented NY and NJ. Because I went to school in St Louis, I had to notify FCC engineers in districts from PA to MO. I also had to give a reason for wanting a license; I was an engineer at the university carrier current radio station, and the radio was required by each of us to set up and maintain transmitters around the campus.
Calls you hear could be someone using what was an original license/call. It is only a possibility, as the call format did not conform to ITU agreements. The FCC had to change user licenses as the 5-year license came up for renewal; my second license was KOG-2427.
By the time my second 5-year license expired the FCC had thrown in the towel on issuing licenses. In the following years, channel 19 and "shooting skip" were rampant... you know the rest of the story.
Users made up nicknames that could be anything, and enforcement of the rules were mostly non-existent.
Numeric-type calls? Invented in the minds of the users... 😀
HTH.