In my latest post, I pointed to a hypothesized second-century layer of mainstream readings in Bezae’s text of John 4:1-42. This layer consists of readings whose distinguishing characteristic is agreement with the majority text (‘BYZ’). Yet the agreement of most readings in this mainstream layer with the early papyrus P66 (ca 200) suggests that it must have been in existence by the end of the second century. 
Of course, there is nothing surprising about a second-century mainstream layer. We might observe what G. Zuntz referred to as “that great common reservoir” of readings from which all later streams flowed.  The difference though, is that one of the streams flowing out of the second century is demonstrably “mainstream” rather than “Western” as Zuntz supposed.
To get a sense of the second-century date by which this layer must have existed, we can plot Bezae’s readings by layer as described in detail in this earlier post. The example below depicts Bezae’s agreements in each of its readings in John 4:1-42, with readings attested by P66 highlighted in red:
[Update: note that the highlighted readings include both initial and probable secondary readings (for a plot of initial vs. secondary readings, see figure 3 at the bottom of this post). Of course, only the latter would date from the second century.]
For reference, my interpretation of the layers from an earlier post is:
- Transitional Greek (8 readings, marked ‘o’)
- Mainstream / Byzantine (33 readings, ‘Δ’)
- Free Greek (9 readings, ‘+’)
- Alexandrian (5 readings, ‘×’)
- African Old Latin (8 readings, ‘◊’)
- European Old Latin (10 readings, ‘∇’)
The axes correspond to the directions of greatest diversity in the per-witness support for the readings. Intuitively, the left-to-right axis seems to register the closeness of a given reading to the mainstream layer, while the up-down axis seems to register along versional or geographical lines. (Keep in mind though that every reading in the plot is a Bezan reading. The graph merely shows Bezae’s agreements per reading.)
It is clear that the readings attested by P66 are clustered over the far-right ellipse corresponding to a (hypothesized) mainstream layer. If we assume a degree of coherence within the readings of this layer, we may conclude that the layer must have existed by at least the paleographically-assigned date of P66. 
To corroborate this result, we can compare the readings of another early witness, in this case Origen, coded in green:
But Bezae’s agreements with Origen are also clustered with the hypothetical mainstream layer at the far right, which is consistent with the result for P66.
[Update: The plot below is color-coded to indicate Bezae’s initial and secondary readings in John 4:1-42:]
[Update: The complete list of readings per layer is downloadable as a PDF or listed here. Look for the “Level” column, which estimates whether the reading is initial or secondary according to the local genealogical principle.]
 I would like to acknowledge Tim Joseph, whose comment alerted me to the potential problem in designating this layer “BYZ”, which could imply that its readings are Byzantine.
 Günther Zuntz, The Text of the Epistles; A Disquisition Upon the Corpus Paulinum (London, 1953) 265. For Zuntz, the two streams are “Alexandrian” and “Western”.
 In a recent post on his blog, Timothy Mitchell mentions an article in JBL that reconsiders the date of P66.
[Correction: Timothy Mitchell has kindly pointed out my bibliographical error in footnote . The P66 article is actually in Museum Helveticum:
Brent Nongbri, “The Limits of Palaeographic Dating of Literary Papyri: Some Observations on the Date and Provenance of P.Bodmer II (P66),” Museum Helveticum 71 (2014) 1-35.
The article in JBL will discuss P75:
Brent Nongbri, “Reconsidering the Place of Papyrus Bodmer XIV–XV (P75) in the Textual Criticism of the New Testament,” Journal of Biblical Literature 135 (2016) 405–437.
Thank you, Timothy!]