What Is This Mark Under The Imprint?

It's a small dot that's the same colour as the image, with a white hole in the middle; located under the imprint

Possibly a registration mark used by the printers to line up the perforator.

Commonly known as a perf pip. There are a number of set configurations and they are normally collected as a block of 8 plus selvage across the central gutter. The block may or may not contain the inscription, The perf pip is often sliced off when the sheets are trimmed.

Australian Perforation Guide Pips
https://goscf.com/t/51767&whichpage=7#466101 br /
https://goscf.com/t/51767&whichpage=9#466283 br /

Itma is correct.

Here is the info from ACSC concerning the per pips that was prevalent of KGVI and early QEII sheets. Four types using a combination of small and large dots.

I had never heard of a perf pip before. It is a type of registration mark?

They are a guide for the perforation process. They are printed as part of the normal printing process and at the same time a holes are punched in the sheets in the perf pip locations. Originally the punches were part of the printing plates but this was a weakness which caused the plates to crack. As a result the punches were incorporated into printing machine. I'm not entirely sure of exactly how they work.

Thanks so much for the additional info. I learned something new! Up to now, I had always thought a pip was a guy who sings with Gladys Knight.

In Britain, pips usually referred to the emblems army personnel had on their epaulettes to signify rank. If I remember correctly, it was one pip for a lieutenant - that would be pronounced as leftenant - two for a captain and three for a major. So using the term for small dots is not too outrageous.

...or those frustrating little things I try and fish out with a spoon,
at the bottom of my Lemon Drinks.