[flyingfrizzle]

If you are talking about the second notch inside of the main motor notch:

My 1973 23' sf has the notched down area inside of the well cut out across both motors. It had twin 20" motors. I could use a single 25" but on mine I am going to close the transom in and make a bracket for a single 30". The single 30" brackets provides more flotation and keeps the power head up out of the water better.

Edit: Went back and looked at mine last night. It doesn't look original. Looks like someone maybe redone the transom set it up to be a twin 25" height at one time. Then at some point someone cut it back down to put twin 20" motors back on it. That may explain the notched down area in my transom.