The single comb was only not allowed because the Barred Rock came along, so most of the birds at the time that were Dominiques (which allowed both a single and rose comb, and the single was very popular) were turned into BRs because of the original standard. That pretty much wiped out a huge portion of the higher quality breeding animals.
Tons of Dominiques today have single combs, both coming through from the original breeding, as well as coming through because the breed is regaining popularity and outside breeds have been bred in to help bring the population back up, most commonly the Barred Rock. When you have a bird that has a single comb but very Dominique like coloring or body type, it's difficult if not impossible to decide if it's a poor quality Dominique or a poor quality Barred Rock (unless you know what stock it came from).
In quality birds, the two breeds couldn't be any different...
Look at the Dominique's posture, almost game-bird like, and the wider, uneven bars. They should not be black and white, but dark grey on light grey. You should not see any flat spot on the back, the back should go down until it breaks to the tail, where the tail should be held as close to 45 degrees as possible.
The Barred Rock is a much heavier looking bird, with feathers further from the body (so it's fluffier), the bars are smaller, and should be even so as you could trace a line through the dark bars and they would line up. The bars should also be very distinct. The back should slope slightly up towards the tail after breaking behind the neck.
These pictures aren't the best for showing the difference, it would be nice to see a great rooster of each breed side by side, and you would really see the difference!
Hope that helps!!
And no, neither of the birds posted are myn, I found them on another thread and thought they looked like pretty good representatives of the breed (though I don't really know how to judge BRs very well), and are both hens.
