Harnack, Adolf. “Prolegomena, § 6. "Die Bearbeitungen der Διδαχὴ τῶν ἀποστόλων und ihre Geschichte in der kirchenrechtlichen Literature" pp. 170-268
“3. Die Διδαχή, die Διδασκαλία τῶν ἀποστόλων, die Ignatiusbriefe und der Interpolator dieser Schriften.” pp. 241-268. Lehre der Zwölf Apostel. Leipzig, J.C. Hinrichs, 1884.
Harnack observes that Bryennios identified similarities between the Didache and some parts of the Didascalia (Harnack 1884, 242). Dating from the last third of the third century, Harnack considers that the Didascalia may have used the Didache as a source or that the two works drew on a common source. Likewise, the spurious letters of Ignatius follow a similar pattern (Harnack 1884, 243). The question which arises in Harnack’s mind is what the actual source could be. The question is made more complicated by the fact that considerable interpolation has taken place in both the Didascalia and the letters of Ignatius (Harnack 1884, 244).
Ussher suggested that the interpolator of the Didascalia and the letters of Ignatius was the same person, according to Harnack, citing the same region, time period, extent of interpolations, and some unusual symbolic usage (Harnack 1884, 246). Harnack illustrates Ussher’s argument in some detail (Harnack 1884, 247-253). Harnack concludes that the idea is credible. Yet it is entirely possible that there were two interpolators and that Pseudo-Ignatius knew the Didache. It is also possible that Pseudo-Ignatius is responsible for the interpolations, including those in the Didache (Harnack 1884, 254).
Regardless of the actual identity of the interpolator, Harnack finds it likely that Ussher’s hypothesis about the interpolator being the same is correct (Harnack 1884, 256). Harnack goes on to make his own arguments for the interpolator being one person, mostly by comparing the different interpolations with one another.
Harnack observes that Ussher’s hypothesis has not been universally accepted (Harnack 1884, 259). Harnack traces the objection of Zahn in some detail. The objection seems to be centered around a view of younger men serving as bishops in some source material and not other places. The objection does not seem to persuade Harnack (Harnack 1884, 261). It depends on changes which were not made, though they could have been made. Harnack illustrates this with several parallel passages (Harnack 1884, 262-263).
Harnack concludes by summarizing the point of view he adopted, very like Ussher’s, described above (Harnack 1884, 266 ff). There are sufficient similarities in the interpolations among these documents that they certainly seem to have received treatment by the same redactor, probably in the 3rd or 4th century.