Yes that's correct. These pictures may help. It's just a matter of drilling a hole all the way thru the bait large enough for a single ball bearing to pass thru. Then I make a recess area with a spade bit for the end caps to set in slightly below flush. The end caps can be made out of metal like the picture shows (brass), but now I prefer to use circuit board material as it's easy to cut and shape, and produces a lower frequency knocking sound. They get patched over with epoxy putty and sanded smooth.
Most rattle baits have a chamber that runs lengthwise along the belly. As they are pulled along at a fast retrieve, the rattles are forced to the rear and don't produce as much sound. These side to side single ball rattlers knock with every wiggle at any speed.