Naturalization discrepancy

[Mommyslilmunster]

Hi, I’m new here and just starting to gather documents for dual citizenship. I was going to use my dad’s line, whose grandfather came here in 1900. The issue I came across is that every census and draft card says Na in 1909 and his death certificate has him listed as a US citizen at time of death, as reported by one of his sons. When I requested a NARA and NYC index search, they can’t find any certificate papers, just the declaration, which he made in 1905. I know from other documents that my ggf couldn’t read or write and only spoke Italian, with all forms completed for him so I’m assuming he may have thought the declaration meant citizenship. I’m wondering if having na on all documents, but no certificate of naturalization would disqualify that line.