I don't want to get too deep into this, but what you said was that if the 870 is already registered as less then 470 mm (18"), that the OP may then install a factory 14" barrel on that gauge.
As there only two possibilities, greater than 470 mm, or less than 470 mm, by implying that he may install a 14" barrel if his registration states less than 470 mm, and by the OP's context, installing a 14" barrel would become restricted. That is not so.
The firearm does not become restricted and so there is no need to notify the registry of a change of barrel length. Notification is only absolute if the classification changes. Even then, you are entitled to a 30 grace period after your change of barrel size.
Yes there are FRT numbers for the different barrel sizes. They are applied when a transfer occurs, but has no bearing on the classification of the gun, not on a manually operated shotgun anyway.
There are members here who have notified the CFC of a barrel length change after the initial transfer registration to never receive a new certificate.
My apologies if I ruffled your feathers. Sometimes it's what it takes to clarify what I here believed to be unclear and somewhat incorrect the way you explained it in so few words.
Perhaps what I should have written was that the barrel length of a manually operated shotgun has no bearing on its classification rather than implying legality.
... and BTW, the Express magnum barrels are physically 18.5" even though Remington may in some cases describe them as 18 inch barrels.
