Can anyone teach me

That would have been an additional transmission oil cooler, usually part of the Snowplow package, and sometimes part of the heaviest towing package.

The snowplow package included a tee fitting with a temperature sensor & "electronic monitoring system" to keep you from overheating the transmission during snow plowing. Really just a switch & warning light.

The additional tube & fin cooler mounted in front of the radiator, and there were hard lines to connect it. I believe the radiator part number he lists has extra holes or brackets for the cooler & lines to mount.

B.