Confirmation of P20S-1C-/./
This coin I bought at a local show in Melbourne on the 13th of July. Unfortunately it has been cleaned and is not nearly as an attractive coin as Geoff Raynes' one but it is definitely a 1920 dot above with a London obverse. This one has the poor obverse denticles common for a dot above - indeed the denticles are missing in the region of the 'N' of OMN - the colon after IMP lines up with a visible denticle and the R is the 'short' London die version. As we tend to look at the 'N' first, I wonder how many of coins of this die combination have been missed because the denticles are very difficult to observe on the 1920 dot aboves.
Besides Geoff's coin being much better struck, there is a significant difference between it and this coin. On my coin there is a small rust spot above the top scroll, which does not appear on Geoff's coin. More importantly, the dot is round and perfectly formed rather than pear shaped as on Geoff's specimen. The difference is so great that I am virtually certain that the two coins were struck with different reverse dies. This of course doesn't necessarily mean that more than one obverse die was used. The mintmark appears to have a 'moat', unusual for a dot above, but common in dot above top scrolls and double dots.
Click on the image to see a 1200 dpi scan. These are the best I could manage at home. I'll see if I can get the coin photographed by a digital camera and post some better pictures.
A scan at 2400 dpi shows that the mintmark on this specimen looks completely different to that of the discovery coin.