Undercar inspections are a good thing on older vehicles. Every car that comes through my shop gets one before the works starts. You might be amazed at the number of things we have found just waiting to cause a problem. We have also found issues ( normally rust) that caused us to recommend not proceeding with any work short of a complete restoration.
glad you found the issue

The hose that rubbed through was actually replaced this year, and routed the same as the old one. I had a hard time finding any half inch hose rated for transmission cooler duty. Apparently the new foreign-made hose does not have anywhere near the abrasion resistance as the original hose. Last night I picked up some 3/4-in corrugated split tubing to surround the entire hose to prevent any more rub-throughs.

is that "T" gizmo the thermostat that partially blocks fluid flow until the transmission warms up ?

Seems like back in the day the “slow” leaks would lead to more damage than a sudden loss of fluid where the vehicle stopped moving…. That said we had an old international travel all with a 727 that quit moving due to low fluid more times than I care to think about. Throw 2-3 quarts in and away we’d go!