Well, something has to hold that gear onto the shaft where it rides on the splines...... I agree, there is no call out in the parts to show a snap ring, but I'd say there is supposed to be one. How about in the service manual? Does it show or state one in there? Either way, I think I'd put two on it and see if it will all fit together with a snap ring. On that style of shaft, it would likely be this style of ring:
SNAP RING XXAS.jpg