The chart makes note of three versions of powder J. J0 is noted for shooting ball type slugs, J1 for 12 gauge guns, and J2for the 16s, 20s, 24s, and whatever else.

The versions of J were different powders. But, if you knew the gun was proofed with any version of powder J, you knew what loads to use. At least, back when this data was current.

Best,
Ted