On the wheel you have, the brake disc was held in place with rubber pieces between the hub and the disc. This wheel I think only works with the 72 frontend. If you have an 81 front end, get any wheel from 74 up and it will bolt right into the 81 forks.
Most of the parts sellers, have a parts look up section. they have copies of the parts fiches that Yamaha uses. I use the one at yamaha-motors.com
Leo