These are high quality stainless steel spring bars. If your intention is to use these spring bars with a leather or fabric strap, you should choose spring bars which are the same width as the lugs on your watch. In the case of the Tudor Black Bay Fifty-Eight this is 20mm.
If you intend to use these spring bars with a steel bracelet with solid end links, it can sometimes be extremely difficult to secure the 20mm spring bars. In this instance, 19mm spring bars can be used to secure the bracelet as they still protrude into the lugs far enough to ensure the bracelet is securely attached to the watch head. You should make absolutely sure they are secure prior to wearing the watch.