When asessing this type of material Jeff,
you have to transport yourself back to a different time.
As today, communication was vastly important,
and the quantum leap the penny postage, and all its
offshoots created was a constant challenge to the GPO.
Seething masses of postal clerks at the London GPO
would be madly rushing to and fro, man handling
all postal material in mountainous sacks.
It must have been clear that mail, at one stage
had to be seperated for assessment of charges.
IS=inland section and FS=foreign section would be
obviously the first in the hiearchy.
Some written advice, hope you can read it
source
"Postal Markings printed matter"
Mr. curt furnau and translated from German.

