Wow, two IBOXes! I only have 1 and a half (mostly a pile of parts! - a production model and a pre-production model with tons of extra holes in it an mostly disassembled
) I'll bet you have made more box joints in the past few years than I have.
The only thing I can add is the compressability factor of your stock- your joint with fingers parallel to the direction of the grain may have been near perfect, but off by just a few thou making it tight but still able to be assembled. That would not likely be the case with a joint having fingers perpendicular to the grain. Wood can compresses (and expands/contracts seasonally) across the grain but hardly at all in the direction of the grain.
Since you are an experienced user I'm sure you don't make any of the common errors, e.g. forgetting to lock the mechanism before cutting, forgetting to unlock the mechanism before adjusting finger size or adjusting micro, then briefly unlocking and locking again (mechanism will jump!), etc., etc. FYI, my initial design did not have micro- I was confident it didn't need it if care was used during adjustment. Chris Taylor at INCRA wisely thought otherwise.
We struggled to add micro to the design until I a light when on in my head and I came up with a simple but elegant method. I had a few other improvements that didn't affect performance but came about after the design was finalized so never incorporated, e.g. totally acrylic (vs MDF) blade and dust guard with dust collection port, quick cam mechanism lock, etc.
I don't drop by here often, but will respond to PMs if you or anyone has IBOX (or Lock Miter Master) questions.