I can see the rationale for a logical numerical progression of absolute numbers, but a scale such as this merely tells you how many are out there, not the numbers typically available which defines rarity for the collector. 1000 collectors and 500 pieces known is extremely rare in terms of availability because most will be held in collections and so unavailable to others. 500 collectors of a piece where 1000 are available will always be for sale in a tray somewhere. There was a book written by Cope and Rayner in 1972 called English Milled Coinage where an estimate of the rarity for milled coins in a given grade was made. This is far more useful than a number representing the pieces available which can not anticipate supply vs. demand and hence to a certain extent, price.