I bought two on ebay, a common size 5" and a baby 2", they are beautifully made, either toolmaker or apprentice made. To make one, you would need a lathe, a mill, a surface grinder and a cylindrical grinder, plus heat treatment equipment for the intermediate stages.
Many years ago at work, I found a strange tool in a job lot of stuff they had bought, and one of our apprentices recognised the general shape as being that of a sine bar, but it was not straight. The body was about 5 or 6 inches long and the curve of the body was about a two foot radius. I did not know what a sine bar was at the time, but I remember it very well, and the lad was not winding me up. I posted on the H S M forum about it a year or so back, but nobody had ever heard of a curved one. I don't remember if there were any means of attaching it to something being machined.