Victor and Oscar for Train IDs of the same service in opposite directions

SECR 65 said:

It has come to my attention that the letters 'V' and 'O' are commonly found to be used by the same route / service, but trains running in opposite directions.

One example is the GWR services on the North Downs Line.
I can recall seeing other examples of this.
I have just seen on TikTok that Grand Central plan to run a new service out of London Waterloo, and again, it's V and O.

Is there a specific reason for this, and why always these letters? Are there any other pairs of letters which share the same trait? Are there examples of V and O in an area where they are for different routes? More generally, are there any other letters which have special properties in Train IDs?

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godfreycomplex said:

Means that it’s going between the former British Rail regions that were in place when headcodes were introduced

xOxx- destination Southern region
xMxx - destination Midland region
xExx - destination Eastern region
xSxx - destination Scottish region
xVxx - destination Western region

Sometimes these letters are used outwith this (for example Kingston rounders are 2Oxx and 2Kxx, and Hounslow rounders are 2Rxx & 2Vxx) but that’s the basic principle

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Harpo said:

xLxx towards Anglia region

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D6975 said:

Within the Eastern region, N was for services to the North East. I think this is still adhered to.

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