Wow, that's old! I looked in the Pioneer service manual, and I can't
find it even in the exploded view. It may have come from an assembly
that's not shown as separate parts, but I doubt any of those would have
been disturbed during a belt replacement. We had boxes of those
mechanisms in our skull orchard but chucked them a few months ago,
running out of space. Wish I still had one or two to look at. Good luck!