Heres an odd one. From a friend in New Zealand with a US 75 Ramcharger. Neither the 3" or 3-1/2" but an odd looking coupler.

So cool to have Mopar friends all over the world.

Also from the Moparts Truck Forum....

"Don't forget spline diameter, trucks were typically larger since most used Saginaw boxes.
Trucks pre 80 or so tended to use Saginaw couplers vs the Mopar style."

Based on the info above that had a 3" coupler on a 79 truck, was the Saginaw coupler a 4WD thing or maybe pre 79 instead of pre 80?