NH test cars

This question could be addressed to other railroad forums, but how did these scale cars work? What is the hatch for on the on the side of the unit shown. Did ACF make these cars?

The hatch was to add or remove weight to compensarte for wear on wheels.
The scale test cars were weight and corrected on a tested scale.
The car would then be send to a place with a railroad scale to test the scale.
The cars did not have brakes (other than hand brake) cause brake use would alter weight.
the Brake pipe just feeds straight through to other end.
Company instructions always dictated to handle the car NOT as last car in train, but to have car with functioning brake behind it.

Thanks (dank U wel) DutchRailnut. I was wondering if that was a handbrake or a wheel for leveling adjustment. Did they have calibrated metal weights to insert into the hatchway?

Alstublieft, I believe they used any kind of scrap metal, the hatches were locked and sealed once car was calibrated.