Who makes transition era covered hoppers?

He is absoulutley correct and Airslides would not usually be used for cement or sand as the cubic capacity of the car (2600CF) and the weight of the material would seriously overload the bearings and frame of the car.

Here is an Airslide:

Note the single unloading bay under the car, they also made an two bay version that held 4000 FG

Here is a standard 1958 CF Covered Hopper with Circular Hatches, usually a retrofit to an old ACF car or was used on a PS 2003 Car.

The first car is a 1958 CF ACF Car with circular hatches, and the second is a 2003 CF PS Car.

Rick J