In Canada, SBS are legal and non-registered so long as they're 26" OAL. In the case of the coach gun and the lever action, their OAL exceeds 26" despite their short barrel length. The quirk about this though is that the barrel has to be made short from the factory. If you have a 35" OAL with a 20" barrel and saw even 1" off the barrel, that becomes an illegal barrel despite the OAL being legal. If you saw the stock off completely and have just a barreled action that still exceeds a 26" OAL, then that is fine. Buying a barrel that was made short from the start - as in no modification to the length after it left the factory - you can replace the longer barrel on a shotgun with this factory short version legally, again so long as it meets OAL requirements. If you want to go deep, a licensed gunsmith may shorten, test, and safety approve a barrel and sell it, or if in doing so he lowers the OAL of his gun below 26", he may register the whole package as a restricted firearm and sell it to a restricted license holder.
>pre-1900s shotgun designs
Not sure how it is in the states but up here, (most) guns with a manufacture date before 1898 is considered an antique firearm which puts it on the same legal level as an airsoft gun from the hardware store. If it's a reproduction like the 1887 I have that was made in china 3 years ago, then it's not technically an antique. Unless it's a flintlock or wheellock black powder firearms in which case replicas of models preceding 1897 are often considered non-firearms.
If i sawed off my barrels illegally of course I wouldn't rush to FBIchan 2020 to blog about it