In the "Angel" (1999/I) flashback episode 5.13 "1943" (aka "Why We
Fight") Angel is summoned by Commander PETRIE from a military
organisation named "Demon Research Initiative". This is an in-joke to
"Buffy the Vampire Slayer" (1997) writer/producer Douglas Petrie. In
season 4 of "Buffy" Petrie wrote the episode "The Initiative", which
gave us the first major glimpse of the military organisation The
Initiative. Also in "1943" Angel gives a naval command code as
"4-zero-7" which was the Buffy episode's airing number: 4.07.
