The only 1965 Mustangs to not have rocker moldings were those with the GT option. If your 65 Mustang is an "A" or "K" code car, built after March 1965, it may be a GT. If not, a previous owner removed the mouldings and filled the holes. The easiest way to determine the latter is to look at the lower rear section of the front fender and/or the lower front section of the rear wheel well for Bondo filled mounting holes.
The next question is - why do you want to do this?