Papyrus Hamburg 1.18 (Schmidt-Schubart)
It is interesting to note that the nomina sacra in the formula, “One is Christ Jesus” (cf. 1 Cor 8.6) includes the word “one” (εις), as is made clear by the superlineal stroke above the (unabbreviated) word. There is a tendency for words associated with name of Jesus to be made into nomina sacra, but this is the only instance that I can find of the word “one” and I would happily receive information about other cases like this.
Acta Pauli 
Note that the scribe writes εἷς in the same way at 5.17, which is in a neutral context: “one of those fleeing.”
Julia: That’s a great find. Thanks. The line is above the episilon only, but it does appear to be a pen stroke and not an imperfection in the papyrus.
I believe that in this case it must be a diacritical mark which would indicate a rough breathing or to distinguish EIC (“one”) from EIC (“into”). A similar line is found above υπο (5,10) and ο (5.1). Elsewhere the papyrus has a diaeresis above υπο and other letters.
Why are you interested in this configuration?
I suspect I am trolling to the meat of your studies.
One God and One Lord? The 1 Cor ref. subordinates the master to the one God. This also is not exactly creedal. As Didache portrays.
The study of nomina sacra (an abbreviated sacred name drawn with a line above it) in ancient manuscripts is of interest to scholars. The studies that I’ve seen do not list “one” as a nomina sacra. Therefore, this post is a contribution to field in that it offers the possibility of new name to add to the list.
This particular nomina sacra would interest you because the formula here derives from the Shema.