I just tried out the opcard firmware on my NK3 Mini.
Flashing went fine:
Successfully updated the firmware to version v1.2.2.
The next steps from the USAGE.md description:
$ gpg --edit-card --expert
gpg: WARNING: server ‘scdaemon’ is older than us (0.10.0 < 2.2.40)
gpg: Note: Outdated servers may lack important security fixes.
gpg: Note: Use the command “gpgconf --kill all” to restart them.
Reader …: [none]
Application ID …: D276000124011xxxxxxxxx
Application type .: OpenPGP
Version …: 11.50
Manufacturer …: ?
Serial number …: xxxxxxxxx
Name of cardholder: [not set]
Language prefs …: [not set]
URL of public key : [not set]
Login data …: [not set]
Signature PIN …: forced
Key attributes …: rsa48 rsa48 rsa48
Max. PIN lengths .: 0 0 0
PIN retry counter : 0 0 0
Signature counter : 0
Signature key …: [none]
Encryption key…: [none]
Authentication key: [none]
General key info…: [none]
Admin commands are allowed
gpg: key operation not possible: Invalid data
gpg: error getting card info: Invalid data
I tried re-connecting the NK3Mini, that did not change anything.
Also this must be my NK3Mini as gpg could not find any card when it was disconnected.