I was the one that replaced Alpha by ARM64 as 7 for Plan 9. 

On Wed, 31 Oct 2018 at 13:44, Ethan Gardener <eekee57@fastmail.fm> wrote:
On Tue, Oct 30, 2018, at 3:37 PM, Steve Simon wrote:
>
> There once was a vax port but i don’t know what its letter was.

Well, 7 used to be Alpha.  I remember someone being very annoyed because he still had Alpha hardware when the Go team decided to use 7 for Arm64.