T-Posts are typically available in 2 versions;

Standard Duty at 1.25# per foot
Heavy Duty 1.33# per foot. 

ST7 T post hooks will fit both however cross-sectional size can vary significantly post to post. Although it's rarely an issue, if purchasing heavy-duty T Posts it's best to bring a T Post hanger into the store with you to make sure they fit.

OLD Tposts

You probably know that very old 2x4's are much bigger than what's sold now, T posts are the same way so if your grabbing grandpa's T posts off the back 40 you might want to purchase new.

Hammered on T Posts

If your posts are old or have been driven in hard it is possible the upper part of it will mushroom and cause problems getting the hangers over the posts. To avoid this use a T Post driver or file off the mushroomed area up top if you crushed it hammering it into the ground.