I consider myself to be somewhat of an expert on the topic of the antique status of the Colt Single Action Army. I am well versed in the applicable sections of the firearms act and more importantly, the criminal code. I have had lengthy conversations with RCMP officers and employees of the firearms lab on this topic, and from this experience I can tell you this:
Despite what many here think, the RCMP Firearms Lab does not make the law. They are, however, responsible for interpreting the law as it pertains to firearms. Due in part to the complex and vague nature of Canadian firearms law, this interpretation is open to being both broad and loose. In other words, when there is doubt as to the law, the RCMP firearms lab employees get to make a decision that effects the enforcement of the law - and this interpretation effects all firearms owners.
The key to understanding the lab's decision on the status of pre-1898 Colt lies in the interpretation of Schedule 1-7 of the Regulations Prescribing Antique Firearms, which in part reads "other than a handgun designed or adapted to discharge..." and goes on to list the vast majority of the common Colt SAA calibers. The firearms lab, as of about 4 years ago, has decided to interpret "designed" as "originally manufactured". Right or wrong, that is the case.
So, for the Colt SAA this means that in order for them to consider a pistol antique (and thus deregister and/or issue an antique letter) the gun must have rolled out of the factory in a caliber no excluded by the regulation, and not currently be in such a caliber. For proof, they will accept an original Colt factory letter, which WILL list odd 45 calibers such as the 455. The sad fact is, other than the 41 Colt, original guns manufactured in calibers not listed on the "no good" list are VERY rare, and considered VERY collectable.
It is true that most of the rank and file members of the RCMP have a very limited understanding of the vague and complex antique firearms regulations - as such, they would rely on their own internal experts at the Firearms lab or NWEST teams. What this means for the owner of a pre-1898 SAA is that what the Lab deems as antique and restricted VERY much matters. It is true that you do not need to produce an antique letter in order to possess an antique firearms. However, should the police catch you with a gun without such a letter, this is the most likely scenario:
You will insist that the gun is antique and no ATT/licence is required. The officer will not know one way or the other, and will seize the gun pending further investigation. The officer will then contact the firearms lab, who will use their interpretation of the firearms act to provide advice to the investigating officer. This advice will be that unless the owner can prove the gun was originally in an antique caliber, it's restricted or prohibited. Best case scenario, you lose the gun. Worst case is you are charged and sent to court, where you'll have ample opportunity to spend huge sums of money on a lawyer to argue your case infront of a judge. That'll be your opportunity to convince the judge that the lab is misinterpreting the law...
You'll often see pre-1898 Colt SAA guns sold on the EE in 38 special or 44 special or some other converted caliber, advertised as antique status but WITHOUT an antique letter. Rest assured, 99 times out of 100 this gun will have been manufactured in a no-go caliber, and wind you in the scenario above. There was recently a US marked gun on the EE, no letter. These guns were ALL in 45LC originally.
As for guns with antique letters, any gun that was lettered before they changed their interpretation was grandfathered. I was specifically told they would not be revoking issued antique letters.
Hope this helps.