Difference in 10" brake drums

OK guys - the picture of the drum on the far left is the mystical 10 x 1 3/4" sbp drum. The friction surfaces on the middle and far right drums are approx. 2 3/4". They are 4 1/2" lbp with the larger register. Overall width is 3 5/8" for both of the larger drums. Doesn't look like the friction surfaces would be the same but they are. The one on the far right is just a meatier drum.