I have another old Ithaca SBT with a similar chunk gone on the other side. In fact, if you look closely at the photo, you will see a crack where the other side is about to let go.
The water table at this point is paper thin because it is deeply cut from the bottom to allow for clearance of the cocking arm (?) for the hammer. If you let the barrel simply drop open after shooting, the cocking arm gives the water table a teriffi whack at this point. Eventually it just cracks and pops a piece out. The one I have had been weakly patched by a bad weld which broke out too. I have been thinking for years of machining a slight recess along the top at that point and silver soldering in a thin patch of metal with a temper that might hold. Bottom line is that the chunk is missing by someone who let the barrel fall open instead of supporting it. It is caused by the design.
