In my regular GNT reading, I noticed this time that Paul uses the same anarthrous construction in 2:2 as he does in 2:15:
μήτε διὰ πνεύματος μήτε διὰ λόγου μήτε διʼ ἐπιστολῆς (2:2)
εἴτε διὰ λόγου εἴτε διʼ ἐπιστολῆς ἡμῶν (2:15)
I believe that the lack of the article in both cases refers not to any specific set of teaching or any specific document, but to teaching or letters in general. This is especially clear in vs. 2 from the context, and there is no reason to suspect any change in vs. 15. The argument sometimes advanced that 2:15 refers to tradition (παράδοσις) in the modern sense of the word is simply fallacious. It refers to Paul’s teaching of the gospel in general.