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Ralph Mowery

In article ,
says...

Most (not all) VOM's did NOT have amplifiers with gain. Therefore,
resistance measurements needed to be using the basic meter
sensitivity. The meter sensitivity and battery voltage put a limit on
the highest resistor value that could be accurately measured.

The meter face usually had the meter sensitivity. In this example:
https://cdn6.bigcommerce.com/s-a1x7hg2jgk/images/stencil/500x659/products/42383/204330/50126_2__62784.1490313691.jpg?c=2
It says "20,000 ohms per volt" (on DC scales) which is the same as:
1 / 20K ohms/volt = 50 µA full scale
You could probably read 1/10th of full scale accurately. So, what's
the largest resistance that you could read at 1/10th of full scale,
using a 9V battery?
R = E / I = 9V / 5*10-6A = 45M
Good enough to measure common resistors of the 1960's. However, if
you tried it with a 1.5V battery, you would get:
R = E / I = 1.5 / 5*10-6A = 7.5M
That's too low, because there were plenty of resistors up to 22M in
older tube sets, that such a meter could not measure.

The Simpson did have a 20 meg mark on the scale. It is almost worthless
at that resistance. About all that can be told is that the reistor is
not totally open. Around 2 to 5 meg ohms is abut the best anyone can
tell close to the resistance. The resistance scale is similar to a log
scale so as the resistance value goes up and below about 1/4 scale the
values start getting very close together.

I have not looked into the 260 working in many years. I know of the 20
K per volt and how it works on the DC ranges, but not sure where it
comes into play on the ohms scale for this meter.

The meter movement is just under 50 uA and has a pot that I think goes
across it so the first step in calibrating one is to set it for 50 uA
full scale.