I don't, other than best guesses. They're all in the Commentary, Prologue section
People always are so eager to solve puzzles without pondering about the merits of doing so, or thinking ahead of what that would bring.
Take Michael Grondin for instance: in order to impress his father he decided to solve the Thomas puzzle, and spent over 4 decades on exercising numerological experiments on it. He could have made a decent translation instead, one that would actually be a solid basis for a forgery, for example. He could have translated words truthfully, for instance the boiling of the fountain, the colostrum instead of the leaven, to name a few.
But he decided to waste more than 4 decades on arguable arithmetic - and I inquired after that, asking him "what if you find all the numerological secrets that are hidden, Michael? That still won't bring you one bit closer to the meaning of Thomas. You haven't published anything on any of it other than some number games that deliver nice round numbers, likely because you fancy nice round numbers - but none of that is going to magically solve the entire puzzle of Thomas, or disclose even a shred of it"
I won't bother you with the response, but instead I'll ask you: what if you find any meaning at all behind IS - what would you be that on? Isn't the best person in the world to know why Harry Potter is called that way JK Rowling, the person who invented him?
You can read all her books and try to conjure a grand plan about it all yourself, but all in all what we the value be in that compared to the magnificent achievement of the books themselves, their great and grand message, the small messages, and everything together?
What would you be doing really other than practice some vain and futile quest, and a finical and trivial one at that?
What does IS stand for? Who gives a damn really, everyone already has his own version of the elephant in the room. Look at the pathetic attempts by Huller to root IS in Hebrew - if anything, that only tells us that Hebrew is the only expertise of the one-trick pony that Huller is. Does he try to base his fantasies on the very first original text? Of course not, he not only lacks any and all qualifications for that but mostly the modesty needed to do so - Huller world be the man who tells JK Rowling what Harry Potter stands for, because that is the person that he is: a claim-it-all, telling everyone what he thinks, as careless and scant the evidence and arguments that come along with that alleged thinking
The thing is, there are 102 occurrences of IS in Thomas, and 3 that say IHS: logion 13, 21 and 90. The superlinear in Coptic functions as a stress, as the vowel e, and in Thomas it covers either IS or HS - never the full word IHS.
I have lost interest in the meaning behind IS a few years ago, when I discovered how incredibly deep the text really runs
Trust me, it is of no significance at all, and it likely points to something like Conscience, Awareness, what Socrates called his eidon perhaps: or maybe it doesn't and it is just a joke to keep us all busy, leading to nothing but the discovery of how vain and trivial the quest into that actually was