threaded is also called an OS shaft. the part of the crankshaft comming out of the engine is only threaded. With a SG there is some thread and on that a 5mm shaft on which you can mount the compleete clutch, with threaded you need an adapter shaft to mount the clutch.

I believe some .12 (side exhaust) Novarossi models can still be bought with a threaded shaft but most engines do come with SG. Do you have a special clutch with an own adapter you can cut off the 5mm shaft making it a threaded shaft.