Advice On 1952 Australian Halfpenny With Different Edge.

Hi robster you prompted me to have a look at the 54's too and there is some (but less severe) grooving in 3 of 18 which is a lower incidence. I found some in the 47's and some as far back as 1935. (haven't checked earlier yet)

I am leaning to very high strike compression causing expanding of both faces, more pronounced at the face than at the centre as the cause.

But why that wouldn't be consistent with all the coins I have no clue unless these are softer planchets, perhaps higher copper content?
Anyone else see this as a possibility?
This seems to be quite common through the years and various mints, has anyone ever seen any disussion of this elsewhere?
I haven't. (But that doesn't mean a lot)

robster I have just looked through 240 Ozzie pennies and found 13 with the same groove which is a far lower incidence than the halfpennies. (too small a sample to draw firm conclusions)
These examples are 1948, 51 x 2, 52 x 3 from the Melbourne Mint, then 1945, 50, 55 and 64 x 2 from the Perth Mint.
It is highly possible from the look of the 1964 examples that they are the result of a different set of factors and are possibly a red herring.
This seems to indicate that the great majority of these incidences are between 1947 and 1955, don't come from London and reasonably evenly originate from Melbourne and Perth.
My thoughts are that it seems a relatively common outcome of the process of striking copper coinage and the only mystery is why there has been no discussion about it. Or if there has why no person here has seen or heard of it.
I only have 80 British and 60 NZ pennies and about the same halfpennies but found no examples so it appears to be an idiosynchrasy of the Australian Minting Process, machinery or planchets.
Hopefully we will learn the facts from someone who knows.