From reading the Kyosho site, ABL would be the hokus-pokus OS came up with after their big black eye from their ABN engines that started pealing their liners. Early FX line engines had a real problem when overheated, the plating on the liner could flake off. A bad thing. It's apparently been fixed in the current FX engines.

For obvious reasons, I personally won't buy a used FX line engine (unless I know the owner, and the age of the engine), because as far as I know, there's no way to tell if it will have the liner problem or not.

In actual use, ABC, ABN, and OS's ABL are all the same thing.