Churchill Omnibus series.

G'day Ron,
I think your answer can be understood by considering
the two sub-sets under the classification of "Omnibus"

There is the Omnibus set of countries participating in the
particular subject commemoration.

Then there is the "Omnibus" set produced by the Crown Agents.
These are the ones with consistant font designs.

If we take the Churchill Omnibus set, there are 182 stamps
in the full set representing 47 countries.
Sierra Leone has by far the largest contribution of 11
these are evidenced as "overprints" they have no design similarities at all

The Crown Agents set will all have similar characteristics.

Here is a set of 1991 Crown Agents Royal Birthdays
that show similar designs/fonts.
http://cjoint.com/data/guheidZWyU.htm
(350 Kb)

This is how I see it, and am open to correction.
HTH