1972 20' stringer question

[Ed]

We have a 1972 20' center console in the shop.

The deck is up and stringers exposed. She has (4) stringers and a keelson.

The stringers sounded hollow - so we drilled an inspection hole and sure enough, they are hollow. Here is the interesting part - when you stick your finger in a limber hole where the stringer meets the transom, you can feel and move a piece of saturated wood.

It appears that the stringers were laid up on a bench and then tabbed into the hull. What is puzzling me is the wood in the aft section at the limber hole. Why was this added and not just left hollow?