You're asking for an exact answer where one doesn't exist. The current law is not made for clear, logical answers. As I've already told you, CBSA has grounds to seize non-receiver externals if they choose to. Right now, that is generally not what they do.
However, if a CBSA agent does choose to seize externals, CBSA would most likely back them up and you'd have quite a fight on your hands. However, though it is illegal to manufacture prohibited devices (replica firearm), it is not illegal (per strict wording in CCC) to possess parts that can be assembled into a replica firearm (can't outlaw wood, after all). Begs the question of why they can even seize replica receivers when it's not assembled onto a gun.
As I've said, the current law is problematic.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."