Rod could you give me more information about
opinion you had on the "O"
I've been trying to educate myself a little on it
To the best of my knowledge.
This is a hammer code, to marry the individual hammer to it's usage in a particular location.
A small Post Office may have 2 similar cancellers ...A and B
A large Post Office may have a counter 40 feet long, and have 6 cancellers, A,B,C etc
As you may well imagine, the control and audit of postal cancellers is/was very important.
They may not be Alphabetical, or in sequence. Many are numeric.
Some cancellers have what appear to be strange codes, these may indicate a Time code when cancelled.