The cap is actually a driver. You put the sleeve on the yoke flange down. Then you set the cap over the sleeve and it rests on the flange of the sleeve. Then you tap the sleeve down or, press it down gently. It is literally paper thin. You can remove the flange after you install the sleeve if you clip it first. There are some good instructions online. These are from SKF but they are the same as the National.
https://www.skf.com/us/products/seal...eve/index.html
From here that yoke looks pretty good! I don't really see a grove. It needs to be polished. Resist the urge to use a coarse abrasive on the seal surface. The surface is actually ground to a micro finish. To bring it back treat it like you were honing a dull knife back to shave with. Fine paper then just paper, or leather for the final polish will work well. No scratches that you can feel with your fingernail are allowed. In manufacturing, the testing of a seal surface is literally done by dragging a weighted string across the surface. The seal can handle a little bit of wear. It wont work long however if the surface is the least bit rough or, the grove is several thousandths deep. The wear sleeves cost $20.