The 1920 is generally considered to be a slightly scarcer than average coin, especially if sought in mint state. While it is certainly scarcer than the 1919 and 1921, its scarcity is somewhat overrated with at least one Sydney mint roll surfacing allowing for numerous mint state examples being available. Like the 1919, the 1920 often suffers from a central strike weakness but to a much greater extent, occasionally being struck with only 6 clear pearls. One should be cautious however, and beware of cleaned VF level coins being passed off as softly struck mint state pieces.
The above illustration shows the band on a softly struck 1920 half penny. As can be seen, the pearls are somewhat unclear if present at all. Mint lustre as always should be an indication of genuine mint state rather than details which may be absent on this issue.