It is a center finder / wiggler for the lathe, they haven't been made for a long time. It is missing the long rod that goes through the gimble. Really cool, but kinda out dated. If you are planing to sell it, shoot me a PM please, I might be interested. Yea, I'm old school like that.
Yes, exactly so, its a Starrett 'No. 65', called a 'Center Tester' in the old catalogues.
I've used mine maybe three or four times in all the years I've owned it.
Its used to centre a centre punch mark on a workpiece, in the lathe.
An example of its use might be in making an eccentric, in which you need a bored hole well out of concentricity with the outside diameter of a part, and the accuracy level required is such that working from layout lines and centre-punch dimples is adequate.
The workpiece would be set up in a four-jaw, and approximated. Then the 'Center Tester' would be put in the tool post, and the point of the short end of the long indicator rod set in the centre-punch mark. The spring holding the 'gimbal' would be adjusted to have the long end of the indicating-rod near to the tail centre.
'Dialling in' the workpiece with this tool would involve slacking/tightening the appropriate chuck jaws until the long end of the indicating-rod remained aligned with the tail-centre as the workpiece was revolved. Alternatively, the workpiece could be clamped to a face-plate, and gently tapped into alignment, with the clamps 'just snug'......when the centring was correct, the clamps would be tightened.
The purpose of the 'short end/long end' configuration was to magnify any error by an order of magnitude, so that, say, an .005 error at the short end would be a viusually noticeable .050 error on the long end. In practice, an .005 error is just barely visible, so if you can read the instrument that closely, your centre-punch dimple would be running out only half a thou.
Needless to say, its a rather obscure tool, and seldom needed.
Its use goes back into history, when good boring-heads for the mill weren't commonly available, so machinists back then would clamp workpieces to a lathe face-plate for boring.
added on edit.......I had to dig out a 1927 Starrett catalogue to find the original 'name' they called that tool....as a bit of trivia, it was priced at $3.75 in 1927.
cheers
Carla