These are thread/size designations. A 2-56 thread is #2 size rod with 56 threads per inch. A 4-40 has 40 threads per inch on a #4 rod. I used to use the 2-56 Quik-Links, but as performance has increased on these things, I only use 4-40s on anything .40 size or larger. There is a tiny weight penalty, but I gladly pay it for strength and safety. I either run solid steel pushrods through Ny-Rod outer sheathes or use Central Hobbies carbon fiber pushrods with titanium ends and Sullivan heavy-duty clevises with jam nuts on the pushrod threads to prevent creep and to lock the treads against vibration.