crujones4life on Pinside claims to have heard from John Borg that Rush was codenamed “Finn.” I’ve updated the OP accordingly.
I came across an old Pinball News article mentioning and explaining the codename for Stern’s original Avengers:
The eagle-eyed amongst you might have noticed the sticker on the underside of the playfield, printed with the words ‘PEEL 164’. ‘Peel’ was the project name for The Avengers pinball, and it comes from the British TV series of the same name from the 1960s, featuring the characters John Steed and Emma Peel.
The ‘Peel’ name is also printed on a label inside the main cabinet as the game identifier. As this was an early Pro model, this may change later in the production run.