Found this on my new core support, looks to have been on there a very long time. Hardware and flared ends look factory, and the brackets match the contour of the core support. Some butchery on the hose ends, but that's expected 41 years of use. Supposedly came off a 79 F350 4x4 with 400 and C6, so I'm assuming this is a trans cooler. Anyone else see one like it?

 

Yep that's a factory cooler.

 

That's probably P/N D5TZ-7A095-A cooler, transmission oil used on 75-up F100-350 with the trailer package.
If I'm right, and you're lucky, you'll find either D5TA 7A095-BA or 703-4156-5 on it somewhere