Lemoyne you have it right all the way up until the point of it being black powder in the original loading.

Nomenclature of that day was pretty simple. The first number of the cartridge was the caliber. The second number of the cartridge was the amount of BP used in the case. And alot of cartridges had a third number. That third number referred to the standard bullet weight. For example the first standard bullet weight for the 30 WCF was 160 grains.

As stated above. Marlin and UMC did not want to use the name Winchester in their rifle and cartridges. So they stayed with current(at the time) nomenclature. Only one problem, they were not using BP in this cartridge. So initially the load was 30 grains of "smokeless" with a 160 grain bullet in 30 caliber. No BP involved what so ever-only in the parent case of 38-50(Ballard) and 38-55.

So they(Marlin and UMC) used the name 30-30, but the second thirty represented smokeless(a stray from the normal classification) powder, not BP. Tom.