Here's hoping I can save someone the hours of searching it took me in the spring.
1978
Trillium 4500, brake magnet part #: 761-5105
10" Hayes, 1988 and before.
Example:
http://www.summitracing.com/int/part...5105/overview/
Managed to find a Raybesto's one and have it ordered locally, but it was a search. I do not know if they are currently made, most RV places wish to swap the whole hub for a Dexter 10".
Cheers...