Was setting up a new CRS309-1G-8S+ and after putting it in SwOS mode, noticed it was running 2.13p (and that the build time stamp was exactly one second after the standard 2.13 version).

Any idea what this version is?

Thanks!

Bumping this as I'd also like to know the answer.

In my case the build time is identical to the second in both versions.

Current Installed Version 2.13p (built at Mon Apr 26 2021 09:21:49 GMT+0100 (British Summer Time))
Latest Available Version 2.13 (built at Mon Apr 26 2021 09:21:49 GMT+0100 (British Summer Time))

I do not know the answer (though I'm guessing that it's the version that's pre-installed piggy-backed on RouterOS or something).
What I do know, is that I actually pressed "Upgrade" there, and then it upgraded to v2.13 (without the p).

What is SwOS 2.13p? See viewtopic.php?p=1021750

It is SwOS's primary version, or you can also call it a backup SwOS version. The primary version feature set is identical to the non-primary version, but we recommend using the non-primary one.