New Zealand (And Probably Other Countries) Plate Blocks

A typical inscription block from New Zealand - let's say a four colour printing - will show the plate numbers as 1A 1B 2C 2A. It seems obvious the the numbers are the plate numbers. My quess is that the A stands for the plate's original state (since there are usually not plate numbers without the letter) and the B, C and so on refer to subsequent changes to the plate. Is this a correct interpretation?

Good day,
It could be cilinder numbers. Each colour is represented by a cilinder and when a cilinder is worn out, the number gets one higher. And sometimes even the order of printing (which cilinder / colour comes first, which second, etc.) can be extracted from these numbers.
There is a lot of information that can be found at following website:
https://web.archive.org/web/20100731025410/http://www.errors.info/content.aspx...q_plates.htm

Erik:

I don't think these are cylinder numbers as, in one case, the inscription block is actually labelled as Plate Numbers, per the following image.


So my original question stands as to whether

1a, 1a, etc are the plates as originally issued

2a, 2a, etc would be additional plates

1b, 3d, etc would be modified (repaired/corrected) plates,

NZ also has cases where the plates are indicated as, for example, 1 2 2 1 with a plate number per colour.

And also there are cases, generally two-coloured printing where the plate number is, for example, 4 a , where the number is one colour and the letter is another per this example.