This May paint a better picture.
Put a dime on top of the Real using the same ruler, perhaps digital calipers would be a nice addition to your hobby. Weight is really a great way on telling the difference in some cases, Most of my Half Real's weigh in around from 19-22 grains where as most of my One Real's weigh in around 44-49 grains, twice as much. Adding another photo showing Half Real with dime and One Real with Nickel size comparisons.I hate to make another reply, but it says on the photo below that the 20mm in diameter are 1 real and not a half?