All of these cars were made in Windsor from 1975-1979, likely they could build the Charger, Magnum and Cordoba all at once. The differences like fenders, trunk lids and bumpers just meant there were a few more piles of parts along the assembly line. I always heard the 1978 Charger was just in case the Magnum styling flopped with the public. Or that they had too many of the old parts with the restyle of the Cordoba. I think the Cordoba restyle in 1978 was to catch up with the Chevy Monte Carlo that went from round headlights to stacked square headlights a few years earlier.
I'm surprised that 1977 in the junkyard had a 9.25" axle, even with the 400 most had the 8.25" axle.