Yes, you are right.
Many of the 3000cft cars were modified to increase capacity.
Taking the Cabot car pictured, it began life as part of series CABX 107-146 built in Dec 1949 by GA.
Similar cars were built by ACF and PSC.
In 1967, CABX 127-146 (20 cars) had their capacity increased to 4079cft by increasing the height by 28”.
In some photos you can see the horizontal weld line along the side of the car.
Of course, all this lies in the future for this list.