in https://bugs.debian.org/820676, David Bremner writes:
The attach test script seems to work OK with GPG=gpg2, but fails with
GPG=gpg; here fails means invokes pinentry.
It looks like the passphrase presetting fails with gpg1
Is it possible that passphrase presets only work for keys that are stored in
private-keys-v1.d/ ? if so, do we need to document this somehow?
gpg1 does not known about keygrips. Instead of the keygrip, gpg1 uses the
fingerprint as cacheid for gpg-agent. The agent's command GET_PASSPHRAE, as
used by gpg1, uses a different cache mode from what gpg-preset-passphrases uses.
Thus even if you replace the keygrip with the fingerprint of the (sub)key, it
won't work.
I'll add
Note, that the tool @command{gpg-preset-passphrase}, which comes
with GnuPG-2, cannot be used to preset a passphrase for this
version of GnuPG.to the gpg 1 man page.
Thanks for the explanation, Werner.
This note might also be worth adding to the gpg-preset-passphrase manpage.