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1970 and earlier 13' models had the notched transom for a 15 inch Short Shaft motor.
http://www.whalercentral.com/viewpage...age_id=155

The transom adapter you show raises the transom 5 inches to allow for a 20 inch Long Shaft motor.
Kent Fabrications seems to have changed their website and basically just tells us to call them.
https://www.kentfabrications.comations.com