Judging by the numbers themselves, I'd guess they're from a census, and indicate the page on which the name is found. New York also did censuses in the years ending with five, so the page numbers could be from a New York or a US census. Elizabethtown County Clerk's office has all the naturalization certificates, filed alphabetically. If you're very lucky, you can also find the Declaration of Intent to Naturalize form on file, but most of them have been lost.