From my research, using the factory parts book. It looks like the safety dowel pin was first found on the 1970 and later coupling body. Before that, there was no hole or pin in the coupling body.
Well, never say never. I just went out to the check the coupling on my 69 Dart and I see that it HAS the safety retaining pin in the coupler body. When I get the chance, I will check my 68
