That is a common issue on the early 70's mainly because of age and inner wood core decay. The outer fiberglass "shell" is separating from wood transom inner support and causes the crack. You can patch it or cover it up but it will eventually come back. We all have varying degrees of this on our early 70's hulls that still have original transom like yours. Run the boat, enjoy it, keep an I on it, maybe a little silicone over the crack to help keep more water from migrating inside. Yours is in a good hiding spot. Our boats are over 35 years old and they get wrinkles just like we do! She looks great and you are really going to enjoy her, these boats seem to do everything well.