You asked Might I find other papers? I'm guessing you mean might there be additional records in the C-files for the 1921 or earlier naturalizations, for which you already have the court records? It is possible, but unknown. The best way to hedge that bet would be to search a name index to Bureau of Naturalization Correspondence at the National Archives, NARA microfilm publication A3388. Unfortunately it is only available on microfilm at NARA, which remains closed. If you found your grandparents' name in there, and it pointed to a C-file number, it would mean additional records were placed in that C-file. Yes, it is very frustrating.
You say you do want to request one of the C-files. If you already have the petition from the court records, look on the back or bottom of that petition for the certificate number. It should say something like "Certificate of Naturalization # _____ issued this day . . .". You can then use that number to request the C-file directly without having to pay for the index search.
I hope that helps,