I just verified Chad's comment..no production line install of sun roof!
And...there is no listing in the mt-87 ""accessories for International trucks parts marketing" catalog published for 1979. If there was ever a "dealer-installed" sun roof kit available from IH, it would have had a p/n assigned and be shown in that extensive catalog.
This stuff was implemented in two ways basically...an aftermarket sun roof kit (very popular back then), that was installed by a local "van conversion" operation, typically with scads of dumdum tape and an early form of "dow corning silicone sealer", cut out with a saber saw, and then the headliner dinged all over in the process!
The "package" vehicles such as a "midas" conversion, the stuff done for IH by good times, etc. Used a similar sun roof kit, but was installed in a professional manner at a "conversion center" before the vehicle was "trans-shipped" to the selling dealer. In these conversions, the headliner /inner liner was removed and cut out neatly with a permanent template. A full roof template was lowered over the exterior of the roof, and holes were drilled and the cutout made from a "jig". Then the raw sheet metal edges were sealed, the sun roof installed in an "engineered" fashion, tested for any water leakage, and then the headliner re-installed with the finisher piece for trim.
I spent many years in facilities where this type of "accessorization" was done for nearly all oem applications. It's still done that way for many of 'em. Has to do with production volume, import/export legalbeagle/duties, cost, etc. We call those facilities today...a "port of entry" or "port", even though it May be a sub-contractor at the intermodal railhead in salt lake city!
Iirc, back in the day, IH operated two "sales preparation centers" in the u.s., coulda been three, I just don't remember! Usually in conjunction with a transportation/cross-shipping hub/terminal.