Well, I can "clearly see" the SD on both these coins, or at least there is some kind of marking where the SD is supposed to be:
I'm not really seeing any difference in size of the effigy either.
As for the counterfeit theory, I don't think 20 cents was worth counterfeiting, even back in 1968. Can't say I've heard of any instances of it.
Have you checked the weight?
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis