I have a 1975 20' sf and this is what the stringers look like where they tie into the transom. The knee or trinangle brace on the center stringer is obviously an add on that I did. Your boat should have the same configuration as mine; one center stringer which is a 2x4 glassed over and two outer stringers that are 12 inches accross the top. These are glass over foam. They run all the way up to the front casting deck. Here's a picture of just the transom area of my boat.
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