Well, I am not sure why 17 equals an 18k lb capacity but here is how the book breaks it down.

13=10k to 15k (varies with year and housing)
16=16k early, 17k late
17=18k early, 18.5k late
18=21k early, 22k late
19=22k early, 23k late
20=23k
22=29k

As far as early/late the only 17 series rear I see that it pertains to is the 17500-01, the early version has 2.125 inch spline axle, late version is 1.97 inch spline. I have a 52 GMC and it has the 1.97 spline. It does not state an early/late transition year.