The point I'm trying to make here is that CBSA sometimes seizes parts because they aren't aware of what's allowed and what isn't. What's significant about my case is that the government ruled, in writing, that airsoft gun parts that would not be considered the controlled part of a firearm, are admissible as toy gun parts/accessories. This doesn't give everyone carte blanche on importation, though it could prove useful to those that have had other parts seized and need to make a similar case.
And no, not having a serial number on a lower receiver doesn't exclude it from being controlled. Whichever is the controlled part on the real gun is the controlled part on the airsoft gun. Therefore an AR-15 upper receiver is uncontrolled and admissible to Canada without restrictions.