I've been doing some more reading on A. K. Warder's theory of meter.
The verses in our Raahula Sutta (SN II.11) are in a (relatively) old
meter called vatta.
Now, there is a normal form of vatta (called pathyaa) plus several
variations on it (called vipulaa).
Warder puts forward the hypothesis that, as time went by, the
composers of the verses became less and less creative and more
conservative.
So, the proportion of the lines they would write in the pathyaa form
increased, and the proportion of the lines they would write in the
vipulaa variations decreased.
This leads him to experiment with stratifying texts according to the
percentage of pathyaa lines they include -- lower percentage pathyaa
= earlier, higher percentage pathyaa = later.
What he finds, he says, is that "the differences of age thus
suggested seem to agree with the general drift of subjective opinion
of which sort of doctrine is earlier and which later. Thus the
more 'rigid' doctrine and the more minute analysis of categories of
existence and psychological constituents are found in verses whose
percentage of pathyaa is higher" (Pali Metre, p. 197).
Now, our Raahula Sutta is made up of 2 verses = 4 lines of setting
(vatthugaathaa), and 6 verses = 12 lines of teaching material.
Interestingly, the 4 introductory lines are all in the normal
(pathyaa) form, whereas the 12 lines of teaching material are split 8
pathyaa, 4 vipulaa.
Undoubtedly this is too small a sample to convince anyone that the 2
introductory verses were a later addition, but it does suggest that
we should keep our eyes open to this possibility.