According to the wiki

https://heads.dyne.org/wiki/usingheads.html

four files are required to burn the ISO ("Once you have obtained all four")

However, not counting the README and 64-bit versions, I see only three
files on the downloads page: 'heads-0.3.1-i386-live.iso',
'heads-0.3.1-i386-live.iso.sha', and 'heads-0.3.1-i386-live.iso.sha.asc'.

When I attempt

$gpg --verify heads-*.asc

with only these three files, I get

gpg: Signature made Sun 02 Jul 2017 05:11:59 AM PDT using RSA key ID
F78637DE
gpg: Can't check signature: public key not found

so I suspect that the public key is the missing fourth file. If so,
where can I find it?

Thanks,

Mason