Mystery Item.

This is a very old measuring instrument associated with my trade. I doubt if anyone can correctly identify it.
I'm beginning to realise the dedication required to blip one photo a day. I had a late gig last night, came home and spent some time working on the computer with a nightcap or two. Got to bed at about 5.00 am, woke at about 1.00 pm and was in no humour to do anything. However, duty called, and after lots of nondescript shots I was drawn to this arcane device which has been sitting in my hall for years.