When the LGR ended, the CFOs told businesses that they had to continue to record {name, address, PAL, make, model, serial, etc} in the CFO log book for non-restricted firearms, but they no longer had to record the certificate number, because the certificate number was the registry certificate the law was getting rid of, and not the {name, address, PAL, make, model, serial, etc}.
Businesses, PALers, MPs, and the media, said "say what?"
And that was mostly cleared up a few months later by Parliament.

I'm not sure how powerful the ATT is. Can an ATT, for example, allow someone to transfer firearms that don't yet have registration certificates, or have had their registration certificates revoked? (e.g. imports, exports). I note the "Storage, Display, Transportation and Handling of Firearms by Individuals Regulations" and "Storage, Display and Transportation of Firearms and Other Weapons by Businesses Regulations" make no mention of certificates.

Bill C42 had a bit about a condition of your RPAL now being that a buyer could, without a separate ATT, go to wherever to take possession of the firearm that had been transferred to you (but not the seller). You still have to wait for transfer approval and a new registration certificate number.