This unusual artifact once belonged to the Crawfurd family of Cartsburn. It is a set of thumbscrews - an instrument of torture - ironically with a cross as part of the key. Thumbscrews were also called thumbikins.
They were described in the Journal of the Society of Antiquaries of Scotland in 1890 who note that they "do not look like they have been used much". The unusual aspect of the thumbscrews is the key which is in the form of a cross which is attached by a chain to an egg-shaped pendant of ivory on which is engraved ihs - the monogram for the name of Jesus. The Journal also states that "the ironwork does not appear to be of Scottish origin. The introduction of the cross and the sacred initials might be said also to imply that it had been fabricated for ecclesiastical purposes."
It is doubtful that thumbscrews were used by the Crawfurd family. They were supposedly given to a member of the family by a "Romish priest" in 1771. Thomas Crawfurd (1746-1791), 4th Baron Cartsburn had travelled widely in Europe and these were probably acquired by him on his travels. They were presented to the McLean Museum (Watt Institution) by Thomas McKnight Crawfurd.
However, there was another curious object collected by Crawfurd with the thumbscrews - a belt of penance. This is in the care of the National Museums Scotland. It is a belt made of iron links with small hooks or spikes attached. It would have been worn around the waist and used as a form of penance. Copyright prevents me from showing a picture of the belt but you can see the object here.
I'm sure these two objects were great talking points when in the hands of the Crawfurd family. I wonder what Robert Burns would have made of them if he had accepted Thomas Crawfurd's invitation to Cartsburn House!