Prefixes I have known (WPX)

Recently received some more wallpaper, attesting to the number of "prefixes" I have contacted on the Amateur bands over 50+ years -- 550 Digital and 600 total.  (There are a lot of prefixes out there.  AA6 is different from AA7, etc.)  Digital contacts include RTTY (radio teletype), PSK31 (phase shift keying), and (mostly) JT65/JT9.

Latest project

What is it?