Technically, you could be in possession of some products in the US, it is the part where you cross the border where you can get into trouble in the US.
Parts 121 is the place that spells out what is covered down there, a stock attaches to a gun, so it is covered. This has absolutely NOTHING to do with Canadian law and importation, this is US law, and you are an alien under their law.
https://www.pmddtc.state.gov/regulations_laws/itar.html