Geppetto,
The Knife includes the IRS axle with D rings. The advantage is the axle is fiberglass (that is why it is yellow) and it does not break as easily as a graphite axle. Plus using the D rings (from a Losi XX) locks the rings in so the rings don't slip on the hubs. You can of course glue or pin the rings on a regular axle. Niftech also makes a great axle. Hope that helps.