I have recently been given a small collection of Pakistani stamps from
the 40s through the 60s and while trying to catalogue them, I have run
into an issue with some of the 'service' stamps.

The 2009 Scott Catalogue shows 3 types of 'service' overprints for
design A3. The 3rd type is clearly different from the first
two.....so, my question is what is the difference between the 'a'
overprint and the 'b' overprint, because in the catalogue they look
alike to me.