Acorn Archimedes 400 series, and the 500 series that followed it (in practice only the A540). The original ARM workstations.
The single board computer Pi naming scheme is based on the BBC Micro (Model A, Model B).
As for the 'plus' - Acorn A7000+ was the last of the Acorn desktop computers to be released.
There is a heavy Acorn influence with Raspberry Pi, for good reason.
Don’t forget that ARM was originally the Acorn RISC Machine.
And the co-creator of ARM, Sophie Wilson, still works for Broadcom - the company which bought part of the remnants of Acorn (Element 14). This is where Eben Upton worked before going off to start the Raspberry Pi foundation.
Jack Lang was involved in the Pi, having also been involved in Netchannel, the STB company which used the Acorn technology and had funding from Hermann Hauser, co-founder of Acorn.
David Braben, co-author of Elite (and author of Zarch/Lander for the Acorn Archimedes) was an early Pi supporter.