#41, #410 or #415 can all fit. It depends on the thickness of the sprockets. 415 sprockets are too thick to fit #41 or #410 chain. Other than that they are interchangeable.
Also 415 sprockets can be ground thinner to fit #41 or #410 chain.
While most kits come with #41 or #410 chain, there is not a set in stone standard with the same chain type each and every time. The factories seem to switch things up a bit as they please. Also, on some kits a #415 chain is too wide and will hit the case, behind the sprocket.
Gearnut pretty well said it all. Some kits come with 415H chain while others have 410.
Out of the box, my kit had 415H and the countershaft sprocket was a little too wide for 410. A bit of careful grinding fixes that.
410 runs best, if you have really good alignment and if it fits the countershaft sprocket. There's almost no side-to-side movement on the sprocket and it fits the rear sprocket better as well. 415H is overkill, intended for 125 dirt bikes and similar.
Apparently, though, 415 is more forgiving if alignment is slightly out.
Yep after getting my kit supplied tensioner installed I realized I was crazy for thinking about a spring tensioner. I only had to tighten the tensioner up twice since I started riding and now its in a happy medium. Just need to wait till my bicycle chain oil comes in so I can oil the chains up properly.