The difference between Newport and Newport Custom:
They are different trim levels, some more chrome on the Custom and different interior seat trim styles & offerings.
Back in the day, you could option up the Newport to be damn close to an Imperial with options if you wanted, but have it in a plain brown wrapper. Which is exactly what my Uncle Joe did in 73. He wasn't a fan of flashy..he didn't want a vinyl top and lots of trim, but he optioned his base Newport with things he deemed meaningful and of utility- the 440, towing package, A/C, PW, Seek Scan Am/FM radio for long trips (he hunted and fished a lot and took long road trips towing his trailer).
Those were the days.