There were also a number of M2 carbines (with M1 trigger paks) brought in to the country as M1s. Even though the rifle is identical to the M1, other than the stamped 2, these will fall under 12.3 class. You have to watch out if you are buying a previously registered rifle which has not gone through verification. Just because the certificate says non-restricted, or restricted, doesn't mean it will be after verification.
As I found out a year or two ago, an extension on the barrel, even if it is rifled, will not count towards the overall barrel length. RCMP insist the rifling be seamless. A full length sleeve will count though.