Diplai in P.Bodmer II (P66)

Last week, I was reading through the Gospel of John’s story about the washing of the disciples’ feet in Greek (chapter 13) and came across a variant I thought I would check in the original manuscript. In John 13:5, the NA28 prints νιπτῆρα as the “basin” in which Jesus pours the water with which he will wash the disciples’ feet. There is a variant here that is read, apparently, only by P.Bodmer II (P66) and a few proto-Bohairic versions: ποδονιπτῆρα (“foot-basin”). I took a look at my facsimile of P.Bodmer II and realized that there is a curious little mark interrupting the letters of the word ποδονιπτῆρα. It is graphically represented as ποδονι > πτηρα. Here is what it looks like in the actual manuscript:

This looks exactly like a diple and I cannot help but wonder if the scribe is using it to signify that the reading is spurious, that there is a variant, etc. I looked a little more and realized that the scribe uses this diple-like sign several times. It became evident that the sign is most often employed where there is a variant. Here are a few examples:

1:38: αυτοις > τι                      variant τινα
2:7/8: ανα > και                       variant omit και
4:24: αλη>θια                          variant αληθειας
10:29: μου > ο                         variant omit μου
11:28: εφωνησεν > Μαριαμ    variant Μαριαν

By no means is this an exhaustive list of the occurrences of this critical mark in P.Bodmer II; I have taken only a cursory glance at the facsimile. My question is this: are these diple an indication that the scribe knew of a variant reading? In a few places, the diple occurs besides words where there is no variant (e.g., 10:25 εργα > α), so the answer may be that it is not used in this manner. Moreover, the insertion of it within the text is odd. Normally, diplai are inserted in the margin and are used for a variety of purposes, such as to signify a quotation, refer to a commentary, etc. But if it is not being used to signal a variant, then what is its function? It is surely not a line-filler since the scribe consistently uses apostrophe-like marks for this purpose and line-fillers occur at the end of a line—they are not inserted in the middle of the text so as to interrupt a word. I have not looked at any of the literature on P.Bodmer II for this, but does anyone else know if R. Kasser, V. Martin, G. Fee, J. Royse or others say anything about these little diplai within the text of P.Bodmer II? Surely someone has said something about these diplai and offered a reasonable explanation. I would be grateful if someone could provide a reference to a discussion of these marks.