I'm having trouble verifying the pgp signature of the emacs 27.2
sources.  I've downloaded

  https://ftp.gnu.org/gnu/emacs/emacs-27.2.tar.xz, and
  https://ftp.gnu.org/gnu/emacs/emacs-27.2.tar.xz.sig

I'd like to verify the tar.xz file before building, but I'm running
into this:

  $ gpg --verify emacs-27.2.tar.xz.sig
  gpg: assuming signed data in 'emacs-27.2.tar.xz'
  gpg: Signature made Thu 25 Mar 2021 07:53:08 AM EDT
  gpg:                using RSA key 0x91C1262F01EB8D39
  gpg: Can't check signature: No public key

And the keyserver I normally use doesn't know about key
0x91C1262F01EB8D39:

  $ gpg --keyserver keys.openpgp.org --recv-key 0x91C1262F01EB8D39
  gpg: keyserver receive failed: No data

Where can I find a copy of the signing key, so I can verify the source
distribution I've downloaded?

Steve