Always go to the source wherever possible and NOT Cornell.edu.
16 USC 551 should be viewed here:
https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title16-section551&num=0&edition=prelim#551_1
It states that 471 was repealed but that other sections and federal regulations are  still effective. This leads us to 36 C.F.R. 261.50(a) and (b) and your best source for that is here:
https://www.govinfo.gov/content/pkg/CFR-2019-title36-vol2/pdf/CFR-2019-title36-vol2-part261.pdf

I’d love to help you further but I am busy dealing with the CA FTB who wants my 93-year old mother to file a CA return even though she has lived continuously in Florida since 1989 but we use my home address to file her federal returns. I have to jump through this hoop of fire every year. Not dealing with this promptly may eat into my soaring budget. See you in the air!