The caps are the buttons that go in the housing. They have a housing cover which is a substitute for the chrome housing. Never seen the cover, I have the chrome housing and chrome buttons and they look good. have had them for a little more than 1 year and they still look perfect
When I bought my fatboy it had the left housing changed and the guy I bought it from gave me the right one to do myself. The chrome housing look great, it wasn't super hard to change-a bit of a PITA getting all the wires to go where you want, but do-able. I'm not sure of the cost difference between the covers and new chrome housings, but if it's under $100 I'd say go with the new chrome housings as they look like they were meant to be there and not an afterthought. Just take your time and follow those horrible directions they give you.