no it's not rare, ptobably quite similar bodies put on all make truck chassis back then -
BlueBird supplied conversion bodies, now they make
the whole thing I don't think there's been a BlueBird bus body on anything but their own frame in a long time - and now they belong to some other corp, doubt they'd even have any info on your bus body
might try
here, might be a similar bodied bus, even some GM
Bill