Thanks for all your input above.
I am in Saskatchewan -- so much for the relatively less stringent regulation here.
At no point was I asked for the barrel length.
The full story is that I asked for an ATT to bring the firearm back from Alberta. My request was delayed and I was called instead because there was no record of any att's issued for the rifle's location at that time. The female investigator calling was interested in how the receiver went from the manufacturer, to the assembler, the address of the person holding the rifle for me. I found her attitude to be combative and condescending.
I suggested that while I was not privy to these movements, I could only assume that they were accomplished by mail, since Canada Post does not need an ATT. I was told explicitly and sharply that only a police officer could transport a restricted firearm, so there must be a trail of ATT's. I replied that I had no clue what was done in the past, but that for my part, I was trying to follow all the necessary rules.
In the end, the ATT was issued at the last minute, and I transported the rifle.
Apparently, I now have to find a verifier to confirm that this rifle is an AR, and is not prohibited for any reason.