There are other ways of confirming that you may have the 1974 S doubled mint mark. First, the date(thin date is doubled mm), there is a difference between the design of the date. Example; where loop of nine ends, if tip is flat not rounded it is the doubled one. There are also distinct differences with the 7 and 4 also. The image in the memorial on reverse is another. Having said that, I believe you have one. The signs are there. Nice!