If you are an 07 or even an 01, you buy the action and barrel, stick them together, you now "Manufactured" a new firearm.It starts by stating "each firearm manufactured or imported"- so that applies only to newly manufactured firearms.
Customer buys parts and has you stick them together, you are no longer "manufacturing". Thats how it reads under today's law anyways.
Both cases you still have a serialized part, but under new rules, sounds like you'll have to mark 1st example again with your info.
I forget exactly, but they did at some point talk about recording "multiple" markings under a category.