I remember you sere looking for theese backrest supports and looking for somthing else i found them i just may use theese slso i want to make a backrest on my keaning post and this wouldnt use any rodholders and isould be cheaper hardware.
http://greatlakesskipper.com/product/16_254-boat-seating-seating-hardware/29471--standard-stainless-steel-helm-boat-seat-support-bracket.html