From a government perspective, it is unlikely that separate "attachment stamps" will make a return as I do not intend to change the Register again.
If these physical items are added, then it will most likely be regulated on the basis that a firearm is upgraded in classification when a certain attachment is attached to it (for example, if a class 2 semi-automatic weapon is fitted with an attachment which turns it into a fully-automatic weapon, its classification would be upgraded from class 2 to class 3, etc).