Trying to figure out leak from diverter (back of spout)

[EThorn]

So I'm trying to fix a number of leaking fixtures in the house and one is the bathtub spout (photo below) that leaks badly out the back where the diverter slot is. It would be possible for me to just block that hole but I want to see if anyone has any better ideas (or might happen to know the name of the manufacturer!).

The water enters from both sides (hot\cold) and, when the diverter is in the default position, proceeds up through the deck spout (otherwise it goes down to a showerhead). I'm guessing it should go around the diverter rod and channel but the problem is that the water comes out the slot near the metallic ball. If you look close (and open the thumbnail) you can see that there is a indent on the diverter rod but I can't seem to find anything that stops the water (i.e. an o-ring or similar). Any idea what I should be looking for?

Thanks!

 

[EThorn]

Update: after buying a few different variety trays of orings, I finally found one that fit in that little notch and still allowed me to shove the diverter rod in. Turns out that was the issue, it must have just disintigrated at some point.