1) All that 10lb of resin = "Potter Putty"
2) Center Stringer = Serves no purpose. Was believed to strengthen the stem because most trailers were roller types back in early 70's. My '78 23' had 1/2 cut 4" PVC pipe running the length (Attached Pic)
3) Evidently, whoever installed the transducer thought the teak pieces would reinforce the cutout hole.
4) Agree with Jorge Nidacore the entire cap