Thanks.....Was pretty sure I heard the William unit as 42, but I'll see if I hear it again.
I managed to get a 2016 version of the LAPD Manual, Chapter 4, more recent than what they have posted online, but it is still frustratingly incomplete. What document actually catalogs all of the radio callsigns?
Emergency Operations Division. They do planning and response for major events and critical incidents around the city. In reference to this thread, it includes the special Special Enforcement Section, which has motors officers.