1. Is it really pl 431?
Yes. It is.
2. How can I tell?
3. Is it the latest tape image available (I've seen plenty of disk
images, but those are already installed)?
Yes. Well, almost.
https://www.tuhs.org/Archive/Distributions/UCB/2.11BSD_patch457/ has a tape that's updated to 457, lacking only the last 12 changes. A most current tape hasn't been regenerated, at least in the archives. A quick search of github shows there's a few PiDP-11 oriented versions but I've not looked closely at them.
4. Is there a howto bring it up to the next patch level document
lying around somewhere?
There's two or three efforts to create shell scripts to apply the patches. I've not looked closely at them all yet...
I've seen Warner's work on going the other direction and that's
fascinating in it's own right, but I'd like to see about patching
up to latest
When my work is done, there will be a github repo that has all the changes applied, one at a time which can be used to generate context diffs or something else that can be pushed to the PDP-11s that can be updated.
Warner