In his 1998 paper Hurtado makes clear that he thinks this to be the original form of the nomen sacrum for 'Jesus.' We see at least two second century writers make reference to the numerological value '18' associated with the two letter abbreviation (i.e. the letter I = 10 in Greek and H = 8). The fact that it is an abbreviation and thus follows the standard pattern in epigraphy (whether in Hebrew, Greek or Latin these two and three letter codes abound) actually lends weight to the antiquity of the other forms given that do not appear to follow the standard Greek abbreviation model.
Hurtado distinguishes IH, which he calls 'the suspended form,' from ΙΣ (i.e. the letters which correspond to 'IS' in Greek) and IHΣ (IHS) which he terms 'contracted form' and writes in a more recent book:
Another curious matter that I regard as also significant is that in the case of Jesus' name (Ιησους) two early approaches were taken in treating it as a nomen sacrum. Whereas in most manuscripts the name was written in a contracted form (the first and final letters of the inflected form of a word, e.g., IC, IY), in several manuscripts Ιησους was written in a “suspended” form, just the first two letters, IH) and P45 (P.Chester Beatty II). A third abbreviation scheme is used for Jesus' name in a number of manuscripts, particularly some dated third through fifth century, involving the first two letters and the final letter, e.g., IHS. But this form must reflect an acquaintance with the suspended and the contracted forms of Jesus 'name, and is a conflation of these other abbreviation forms. As Paap put it, this three-letter abbreviation seems to originate in the suspended form which was then “fertilized” by the final letter of the contracted form. The suspended form seems basically to have gone out of usage soon after 300. The contracted forms (IC, etc.) are attested about as early and became by far the most favored way of writing Jesus's name. [The Earliest Christian Artifacts 2006 p. 112]
Hurtado can only imagine a scenario are developed from the Greek name Ιησους. Thus they are either first and second letter and first and last letter abbreviations.
Nevertheless Hurtado fails to mention that the late second century Church Father Irenaeus makes clear that the 'true Church' (i.e. the tradition he shared with earlier Fathers) knew that Jesus's real name was written in Hebrew with 'two letters and a half.' The fact that Irenaeus goes on to say that the Hebrew word baruch was similarly 'two letters and a half' even though, as we know, baruch actually spelled with four Hebrew letters, clearly affirms that Irenaeus is referencing some sort of nomen sacrum convention in Hebrew which was transposed into contemporary Christian writing practices in Greek.
The explanation that most scholars have come up with to explain Irenaeus's enigmatic reference to a 'two letters and a half' designation for the founder of Christianity is to assume that he was familiar with the form of the name that appears in rabbinical writings - i.e. Yeshu (ישו). This is certainly going in the right direction. While Irenaeus's argument is now ultimately quite confusing, this much can be certain. Irenaeus argues that the proper rendering of Jesus's name does not come from the Greek name Ἰησοῦς but the Hebrew name Joshua (). Moreover while some have tried to argue for the first letter yod (י) as the half letter because of its size, Irenaeus makes clear that a group of Christian heretics called the 'followers of Mark' must have understood that the third letter vav (ו) was the 'half' letter.
In other words, and this is quite significant, both the name Joshua (ישוע) and baruch (ברוך) could be rendered as Hebrew abbreviations if - as was natural - their first three letters were employed but the last letter in the abbreviation - the letter vav - was designated a 'half letter' (i.e. ישו and ברו). Since we know that the specific form ישו was widely preserved not only by Jews but Syriac speaking Christians as a short form for Joshua (i.e. the name Ἰησοῦς in Greek), there seems to be something substantive in Irenaeus's arguments.
At the very least, it would seem natural to any outside observer that even if we were to still hang on to the name 'Jesus' as the proper name of the founder of Christianity, the nomen sacrum ΙΣ is not the first and last letters of Ἰησοῦς but rather an abbreviation of the Hebrew name ישוע in Greek letters. The idea that Christians just invented the so-called 'contracted form' out of thin air - i.e. deciding to take the first and last letter of a Greek name rather than the standard practice of first and second - is nothing short of irrational. The more likely explanation then is that just as the nomen sacrum IH was an abbreviation (i.e. the suspended form) of the Greek name Ἰησοῦς, ΙΣ is the equivalent form albeit transliterating the equivalent Hebrew name.
Irenaeus tells us that a Christian sect called the Valentinians not only treasured the Greek name Ἰησοῦς but appealed to the numerological significance of its short form IH. Yet when he makes the argument that the real name for Jesus exists in a short form preserved in 'Hebrew' - i.e. ישו - we have not yet quite uncovered the origins for ΙΣ. Rather we have only established the strong early attachment to Jesus's original name being rooted in Hebrew letters. In order to argue that the nomen sacrum ΙΣ specifically went back to an attempt to transliterate Jesus's Hebrew name we have to delve much deeper in the existing Christian record - back to two curious interpretations of Matthew 5:18 preserved in a text related to the writings of Irenaeus.
dating back to conversations with his grandfather, Gaston Frank. "He said we represent one of the last descendants of the Frankist Jewish faith in the world," he muses. "I grew up thinking that our family was something like the Last of the Mohicans."