Alan, while awaiting the answer from those more knowledgeable, see if you
can borrow a Machinery's Handbook. Most machinists and mechanical
draftsmen will have a copy. It will explain more than you want to know
about threads. And, the American Standards Association (ASA) sets the
standard on screw threads and a whole bunch of other things as well.

Alan Horowitz wrote:

I've got an old screw pitch gauge that I inherited. There's no mfr's
name marked on it, just "Number 137".

the individual leaves are marked with two numbers each, but these
numbers are not reciprocals. The first number is always a small
integer which I presume is threads-per-inch.

the second number is always a decimal.

Any idea what the second set of numbers are?

pairs of numbers are
~~~~~~~~~~~~~~~~~~~~
60 - 0.021
48 - 0.026
40 - 0.032
32 - 0.040
30 - 0.043
28 - 0.046
26 - 0.049
25 - 0.051
24 - 0.063
22 - 0.058
20 - 0.064
19 - 0.067
16 - 0.080
14 - 0.091
13 - 0.098
12 - 0.107
11 - 0.115
10 - 0.126
9 - 0.142
8 - 0.160
7 - 0.183
6 - 0.215
5 - 0.259
4-1/2 - 0.284 (sic)
4 - 0.320

looking at the leaves, "depth of thread" in inches seems the most
plausible to me. This item would date back to the 50's, I don't
believe there was too much metric action in American machine shops at
that time.

I'd try _measuring_ the depths with a micrometer, but I don't know
where to measure to/from. How is thread depth defined? And by the
way, who sets the standards on how these things are measured?