Joe,
To make matters worse, they'd dropped all documentation of it in the years after 1983. So, please try to find a pre-1983 Kirby service manual which covers all Kirbys from the 505 to the Heritage 1HD, which includes the Handi-Butler documentation. As with the Handi-Butler and its belt (mid-1958 and later units only; the earlier ones used the same soft gray belt #159056), the vintage service manuals are also rare as hen's teeth.
~Ben