I find the 54-59 (as you will notice from the picture) has a wider part only at the top & then a round shank for the remainder of the blade.
The Osborne 42-52 (same link) is what I have used before.
IMHO I think of it as more the traditional shape of a saddlers awl (as described by may others here with much more experience than I).
The Bob Douglas awls everyone (including me) rave over have the diamond shape the full length of the blade.
Remember that ANY C.S. Osborne, Dixon, Tandy or most other awl blades you buy will need to be sharpened & polished with a strop to get it to the most efficient condition.
As EVERYONE that has spent the extra money (between 2 to 3 times the price of the other brands listed above) the Bob Douglas awls are bar far the very best out of the box!