Australia 1920 penny

I've noticed several variations on this 1920 australian penny :

dot above bottom scroll
dots below bottom scroll and above upper scroll
without dots
no dot after "PENNY"; Proof
dot below bottom scroll

Can anybody explain the differences ?
If possible with pictures ?

http://monnaiesetvarietes.zxq.net/FICHES/Australie/AUSTRALIE%20one%20penny%20Georges%20V.pdf

 That is the site of 'cobra' Numista member.  

Thanks !!

So this is one without the 2 dots ?

  I would say yes

The penny in the photo has mildly curved bases to the reverse lettering. All 1920 CBL pennies are dot below lower scroll and around half of all 1920 pennies have CBL.
For 1920 flat based lettering (FBL) there are dot below, dot above, double dot (below lower and above top scroll), dot above top scroll only and no dot. The dots below are the most common followed by the dot above. The no dot and double dot are scarce and the dot above top is rare (perhaps 1 in 200 of the 1920 pennies). Also scarce is the dot below FBL with the English obverse. The dot above with English obverse is extremely rare and fewer than 20 are known.

Also this site
http://www.triton.vg/ozcoins.html