|Vintage Cast Iron Cookware was at one time, cast in sizes to fit the tops of wood cook stoves of that era. Many skillets are cast with heat rings. Often times a skillet for the stove was provided with the purchase by the stove manufacturer.
Other Numbers or Letters were simply mold numbers to easily identify a defective mold when large batches of cookware was poured in the foundry process. If a piece was defective it was an easy way to trace the defect back to the mold. The master mold was either corrected or destroyed.
The round covers in the tops of old wood burning cook stoves, (aka plates, disks, stove lids, lid plates, etc) could be lifted out of the cook stove surface (using a lid plate lifter), The plate could be removed when more direct heat was required. The iron skillet could be placed directly over that hole, and directly above the heat.
These removable stove top lid plates came in sizes that used references such as # 8, #9, etc. Hence corresponding sized cookware carried a similar cast marking.
More modern and unmarked iron may not carry the No. reference to help you determine your capacities. Some stand alone numbers, may be mold numbers and be unrelated to size or capacity. In that case you may have to go off the top diameter and depth to determine capacity,
In the below set of charts provided. You will find most conversion information that may prove helpful.