I do know that member up here SeaCratf84 & Parrott re-done a 60's moesly bowrider hull and he also found wood in the stringer such as you did. It was talked about up here and no one could really figure out why it was in there. This was on a original hull that was never cut on either. Like Big said it could of been added as yours dose look to be redone already before but there is a chance it could be original also. One of those moesly hull only deals. The stingers in them are different than in the 70's models. I will try and link the other hull if I can find it.
Edit: Here is the link:
http://www.classicseacraft.com/commu...=25831&page=12
Post #27 "Does anyone know why there is a 1"x4" running the inside of the two inside stringers. You can see it in the pic of the stringer above. It has mat on the back to help glue to stringer. Not sure what the real purpose here is????????"