I think you are seeing a combination of two different vortex breaker designs.
The first is a crossed plate inserted into the throat of the outlet pipe in a tank; the length of which is roughly 1.5 times the diameter. This works best if the outlet pipe is on the side of a tank. A design guide is "ANSI/HI 1.1-1.5-1994 Centrifugal pumps for Nomenclature, definitions, applications and Operation." No design dimensions are given, pointing out the "seat-of-the-pants" origin of this device.
Alternative:
If the outlet from a tank is on the bottom, a baffle plate can be placed over the mouth of the pipe. The rough dimensions are as follows: The outlet pipe diameter is "d" (known), the baffle plate diameter is "4d", and it is placed a distance "d/2" from the outlet pipe. Installing the baffle plate above the pipe opening will simulate a larger intake area, thereby reducing the vortex and air entrainment.
The crossed plate will work okay for a bottom-mounted outlet pipe too. A crossed plate with a cap sounds like a home-brewed solution to vortex breaking.