My mate just dropped me some pics and wondered if i new what it is off...
I am not sure but I suspect the collective will possibly have some answers ??
Ornamental turners milling sub spindle its old
i have seen these style of attachments but we tend to make them in Australia as there aint many Holtzapffel lathes around here
these are the things i noticed
Round leather Belt drive
Cast Brass construction
and can pivot on the spindle not just up and down
Carefully clean it down it should be stamped with a makers mark