I have zero knowledge of lathes and woodworking tools, but pretty good with Google.
Found this, looks like a similar tool:
http://www.supertool.com/forsale/dec/t2.jpg
Doesn't say what it is, the tool on the bottom is called a Spoke Shave from what I gather. He didn't identify the tool that seems to be like yours. Maybe ask him at the supertool website?
Also found this, shows and describes a Sizing Chisel or Sizing Tool.
http://www.fullchisel.com/alburnam15.htm
Go near to the bottom of the page.
And this is an example:
http://i42.tinypic.com/25upto3.jpg
Problem is, this is designed for use with an independent chisel from what I can tell while your curved hook is the piece that has the handle and has an integral chisel.
Maybe yours is just a variation of that concept?