Right, but this is not conclusive until you know the statutory meaning of all the words in this sentence.
In 26 USC 5841, Federal law states that “[t]he Secretary [of the Treasury] shall maintain a central registry of all firearms in the United States which are not in the possession or under the control of the United States.”
Why aren't the feds asking you to register your grandpa's old broken-down .22 with the Secretary of the Treasury? Because in 26 USC § 5845(a), the statutory definition of the word "firearm" is set forth, and it is clear there that they don't mean most of the rifles and handguns in the US, but they do mean poison gas, silencers and land mines.
If you stopped reading the law at 5841, as you have here with section 61 of the IRC, you might not come to that conclusion and erroneously think that your land mine or silencer didn't need to be registered with the government because it's not a "firearm" in the conventional sense, and think that your grandpa's old broken down .22 rifle did, because it is.
What is meant by "all firearms" here in legalese is not the same as what a plain-English reading of the sentence would suggest. And the legalese definition is what matters here, not the plain-English definition.
The same goes for 26 USC 61. The term "whatever source" does not mean "whatever source" if the definition of the word "source" is set forth elsewhere in the law, just as "all firearms" in 26 USC 5841 does not mean "all firearms" because of the definition in 5845(a).
In the annotated United States Code, in the cross-reference for Section 61, we find the following:
Income from sources -
Within the United States, see section 861 of this title.
Without the United States, see section 862 of this title.”
Section 861 and its regulations set forth the definition of the word "source," just as section 5841 in Title 26 sets forth the definition of the word "firearm" for the purposes of the respective statutes.