I thought so too until I put one of my steel protractors on them a
little while ago. I couldn't fit my vernier protractor in there, so I
had to use a sliding-leg protractor with no degree marks. It
definitely looked sharper than 90 degrees.
But I was working half in the dark -- my light fixture just went out.
<g>
I'll take a closer look this weekend.

Hmm. I will remeasure the Vee angle. These measurements are a bit
clumsy to make, and errors are a danger. I think I'll try some
variation of the two rods (different diameter penetrates to different
depth in the V) approach.
I will also take some pictures and measure some dimensions.
I found no casting numbers anywhere. The only numbers found are the
numbers 68 and a star, both hand-stamped into the machined flat
adjacent to the Vee groove. I interpret the 68 as meaning that the
rest was made in 1968, and the star as indicating who made it, or
perhaps what part of the manufacturer's line it went with.
Joe Gwinn

You could trim a small piece of sheet metal to nearly the correct
angle, press the straighter edge against one side of the internal vee
and scribe a line parallel to the other side.
Measure it, construct a more precise sheet metal template at the
suspected angle with a vernier protractor, and see if it fits
light-tight.
Can you tell that I have many years of experience making precise
measurements with office supplies and rinsed-out soda cans?
jsw

On Sat, 15 Jun 2013 12:53:04 -0400, "Jim Wilkins"
I checked my SB again, with some light this time. <g> It is 90
degrees, not 92. Apparently the protractor was slipping as I tightened
the nut.

Following up to my own post, I performed the rods measurement using
five different test rods.
Details: Clamped the rest upside down in a vice, so the base is on
top. Cleaned machined surfaces with acetone and a razor blade.
Clamped a magnetic base to the flat, with arm and digital indicator
over the Vee groove, with indicator probe moving vertically and more or
less perpendicular to the plane of the flat. Installed the 0.500" rod
and zeroed the indicator. Took indicator measurements on five rods,
being 5/8, 1/2, 13/32/ 23/64, and 5/16 inch diameter. Fitted a line to
the data, and also used a formula on measuring tapers using discs of
various diameters from Machinery's Handbook (27th edition, page 715, in
Angles and Tapers).
The consensus answer is 79.7 degrees included angle; one assumes that
the target was 80 degrees. One would assume that people don't sweat
getting Vee-groove angles exact on steady rests.
This measurement is likely accurate enough to exclude the 75 degrees of
Harrison lathes.
By the way, after I cleaned and inspected everything, I think I misread
the hand-stamped number. I had read it as 68, but I now think it's
actually 89, as in 1989.

I'm not criticizing you, but here is a good place to mention this:
I was trained in analytical chemistry, where the goal is to find and
eliminate error sources and make one very accurate measurement with
the limited sample and time available. Later as an electronic test
engineer I practiced solving real-world examples, guided by a former
Keithley Instruments genius Ph.D. of the ponytail, sandals and VW bus
persuasion.
Two useful principles I picked up were that scattered data warns of a
procedural error and shouldn't be blindly averaged in, and to keep
track of the accuracy limit of each measurement. We converted accuracy
limits into parts per thousand to make correlating different types of
measurement easier.
For example if you measure with a 1" dial indicator the best you can
achieve is one part per thousand, half a tick at either end. If your
readings range over 0.1" your accuracy is at best ten parts per
thousand, or 1%, and the final result shouldn't have more digits than
the least accurate measurement validates.
A good sanity check of your height vs width measurements is to see if
they converge to zero at the bottom.
jsw

The plotted data is quite straight. The standard deviation of errors
(deviations from the fitted line) for the five data points is 0.000,431
inches, which is reasonable, as the resolution of the indicator is
0.000,5", and only 5 measurements were made.
If one assumes a uniform distribution of errors over a 0.0005 inch
range, the expected standard deviation if one made many measurements
would be Sqrt[(0.0005^2)/12]= 0.0001443 inches, so the measured fitting
error is about three times that from indicator resolution alone. This
is most likely due to the accumulation of small errors, such as the
indicator axis not exactly perpendicular to the plane of the flat on
the rest base, and the low number of measurements.
But more to the point, it isn't obvious that added accuracy will tell
us anything more, as the ultimate intent is to tell which manufacturer
made the lathe for this stead rest.

The digital indicator has a rectangular LCD as its display, with
0.000,5 as the least significant digit. It is a Mitutoyo code 575-123
"absolute digital indicator", bought used.

It won't go to zero because the indicator was arbitrarily zeroed on the
1/2 inch rod. The point of the V is not accessible, because the
V-groove has the typical slotted bottom where the Vee point would have
been.
PrecisionmachinisT makes the point that it's better to have the
included angle be slightly too small, as the jaws will spread slightly
and yield a firm fit, versus the wobble if the V is slightly too
obtuse, so the 0.3 degree delta may have been intentional. The old
lathe makes made these kinds of tweaks all the time.
Joe Gwinn

Could be, a kinematic fit may be good enough for the steady rest since
it doesn't slide and wear.
The bottom of my South Bend steady was scraped and frosted but the
micrometer stop's vee groove finish would be unimpressive on a Chinese
part.
jsw

There are some scraping marks on this steady rest as well. But they
don't look like someone worked on it all day.
If you recall, I got a 70-degree included angle conical mill bit for
making Vee grooves to fit the Clausing. The resulting grooved blocks
are quite stable on the ways, not being in the least wobbly.
Joe Gwinn

A kinematic fit refers point or line contact, the minimun needed to
prevent wobble. A 3-leg stool is a good example. Unlike a large-area
contact they wear quickly and may deform under heavy load.
jsw

True, and often used in precision instruments. A common variant is
elastic kinematic mounts, where the line and point contacts are
flexures versus rolling or sliding.
The problem with non-flexure kinematic mounts is their sensitivity to
wear, which is why they are rare in machine tools. Scrape-to-fit
methods yield area contact (this being defined in practice by the
number of contact bumps per square inch). Such surfaces are orders of
magnitude more wear tolerant than the point and line contact of
rolling-element kinematic mounts.
Coming back to steady rests, the mystery rest does show signs of having
been scraped a bit. This may have been frosting (for oil retention) or
for looks, or for real - it's hard to say.
Joe Gwinn

79.7 degrees internal will likely flex enough to comform perfectly with an
80 degrees external once all is torqued down.
At 80.3 degrees, it would never conform regardless of how hard you tighten
things.

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