This steady rest came with a Clausing 5914 that was being parted out, 
and I bought it for my 5914.  While this rest does not look like a 
Clausing steady rest, and the fit is not exact, it is certainly workable.
  
(Unlike what came with my 5914.)  But it would be interesting to know 
what make and model of lathe this rest was made for.

In the companion photograph, "Mystery Steady Rest.jpg", the background 
is a one inch grid.  

The material is cast iron for the clamp and body, and steel with brass 
tips for the three guides.  The guide clamp screws are steel, and the 
guide adjustment knobs are aluminum.  There are no casting numbers 
anywhere.  In the machined flat next to the V there is hand stamped 
the following: "* 89".  I take this to mean that the lathe or at least 
the rest was made in 1989.

The bed profile is the traditional Vee-Flat, as used on many lathe families.

The top of the rest is 10" above the flat, the central hole is about 
2.5 inches in diameter, and the V-Flat base is 5" by 2".  

The height from the flat to the center of the hole is 6-1/8".  Fortuitously, 
this is the same as for a Clausing 5900-series lathe.

The clamp that secures rest to bedrails has two machined widths, being 
3.084" and 3.630".  One assumes that this is perfectly suited to go 
between the bedway rails.

From the center of the V to the start of the base flat is 3-3/4", and 
the flat is 7/8" wide.

The included angle of the Vee is nominally 80 degrees (actual is 79.7 
degrees, measured with a series of precision ground rods in the Vee).

Now, this is the mystery.  The included angle of a Clausing Vee is 70 
degrees, not 80 degrees.  South Bend uses 90 degrees, and Harrison 
reportedly uses 75 degrees.

My first thought was that this went with a smaller Clausing lathe, 
maybe 4900 series, but the 80 degree Vee angle argues against that, 
as does the small center hole.  Also, Clausing castings usually have 
a number cast in, and this was Clausing's first question.

Joe Gwinn, 22 June 2013