The slot can be used either for the carpet nozzle or the floor brush...it’s a place to store one while you are using the other. I’d recommend getting the brush for floors as the carpet nozzle does not have any wheels to prevent it from scratching bare floors.
Indeed, I think the brush strip for the upholstery nozzle just snaps in...but why bother when you can get a brand new set of white Eureka tools on eBay for about $20? Look for Eureka Attachment Set 60D.
Some old vac shops may also have old stock of original Eureka attachments....try calling around!
Pretty Princess you got there! Enjoy!