It must've been brute-forced. According to this one old brute forcer I found [1] but have not thoroughly vetted, it would've been one CPU-day of effort on 2012-era hardware
Adding onto this: I was able to use https://github.com/cathugger/mkp224o to generate my own v3 address with chosen 7-character prefix in just under 1 hour on my i7-8700k
I imagine an implementation that leverages a GPU could go much faster.