A Few Terms I Need More Information On

kevin_v I see you are in Canada so here are a couple of Canadian examples.

On March 11, 1964 Canada issued a 7c definitive which was used primarily
for airmail letters to the US.
On July 14, 1964 the rate of air mail postage to the United States was increased to eight cents for the first ounce.
In order to have stamps available in short notice the Post Office Department had
the existing stock of 7c overprinted with the number 8
So it was overprinted and revalued upwards.
Finally on November 18, 1964, a new 8c stamp was issued

Here are the 3 jet stamps of 1964

Canada Scott/Unitrade 414,430,436




Another Canadian example would be Official stamps used by federal departments
throughout Canada.
Instead of issuing specific Official stamps the Post Office Department had
existing definitives overprinted from 1949 - 1950 with the letters O.H.M.S
(On His/Her Majesty's Service)
and between 1950 to 1963 with the letter G (Government)
So this would be an overprint due to change of use.

Scott/Unitrade O38 overprinting on the definitive Scott/Unitrade 334

thank you lithograving for those Canadian examples. That is why I love these forums. Always learn something new.

Why not just think of all of them as "post-original-printing additions"?
Long time ago, I did a sideline collection of them. I think they look best on mint stamps (although the purpose of some of them is to make the stamps "used").

That will not cover it. This thread shows a stamp that, without doubt, was not overprinted or surcharged whatever way you look at it. It was a new printing for which an extra cylinder was used. Your definition would cover this stamp as a detail was added to the original printing.

https://goscf.com/t/78198