Most of your links are broken. I don't know if this is the same nonsense I researched a couple of months ago but there was a similar rumor floating around the web back then related to the same 'firearm registration on tax returns.'
You do understand that 99% of Title 26 (Internal Revenue Code) is implemented by 27 Code of Federal Regulations (27 CFR)? 27 CFR is all Alcohol, Tobacco, and Firearms (BATF). The other 1% of Title 26 applies to Congress and federal employees.
Since the manufacture, sale, and distribution of alcohol, tobacco, and firearms is a privileged activity (not a Right), regulated by the federal government, those people and corporations engaged in those revenue taxable activities
are required to return
(as in 'file a return') of a portion of their income from said activity in the form of an (income) tax.
It only follows that certain questions related to firearms would be on taxpayer forms related to revenue taxable events triggered by Title 26 since 27 CFR concerns itself with alcohol, tobacco, and firearms.
I suspect that when you look closely at such legislation, if true, you'll find that it doesn't apply to American Nationals outside the jurisdiction of the United States (federal government).