I recently worked at a Police Station alongside two firearms officers and asked about certs.
This was as I am sure you can imagine a long conversation!
The up shot, no pun intended was where ever they were in any doubt take the guns first last and always.
So best practice, belts and braces so to speak get the cert and leave them in no doubt.
Still doesn't really help, if an officer thinks it's been mucked about with all the proof stamps and certs in the world won't help.