> I would be really interested to know the oldest example of uniform 16" 
> spacing.

I would as well.  I do remember one particular house I worked on had the 
uniform
16" spacing, and had been built in 1887 (it was still owned by the same 
family).
Nothing stands out to me to suggest this was anything but standard in 
the Victorian
houses here in Sacramento.

> My guess it was developed by trial and error and came to something
> near that standard in the early 1800's and as a rule somewhat later
> say 1920 or so.

There was a discussion of the 16" standard on a contractor's list not 
long ago
and one fellow claimed it was a standard developed in England and had to 
do with
the demands of plastering walls -- the spacing kept the walls from 
flexing and
disturbing the plaster's surface over time. The earliest builder's book 
I have
dates from 1911, and 16" is the standard there.

Mike in Sacto