Pakistan Official Stamps O67-O67A ? Help

Doing inventory of Pakistan stamps and its the Official section, Scott 2009 cat. makes reference to overprints type a and b. Have some difficulty, as a novice, understanding this nomenclature and for ex: O67-O67a...Following pics might help ? Thanks !
Please excuse the slight scanning distorsion - I am new at a handheld scanner.

Hi Renden
The Official overprint of course, refers to the SERVICE overprint. The surcharges are not counted in this instance.
I see in my 2003 Scotts that there are four different SERVICE versions. Each version has a variation in the size and shape of the letters. For example the first version appears to have letters that are quite squat in their appearance. Each progressive version gets a little taller. The SERVICE on the example you give looks to be overprint C or D. So you would then look to see which stamp you have and which overprint would apply, since D was applied later than C. Your example stamp is a 1954 Scott #69. It was overprinted with the new currency in 1961, and is listed in the Official section as SC#O67 also dated 1961, so, your example of the Service over print is version C.
Simple, no?

Thanks James. The A18 with 1 PAISA would be Scott #123 and not an official stamp.
Appreciate the help and yes it is always fun to have a challenge !

Yes, that's correct, regarding the non official stamp. Sorry, neglected to mention that one. Uhhh....just checking to make sure you were paying attention...

Thanks again James and yes.......had to do a "reboot" on that one - LOL