The rare varieties are certainly not listed separately, at least not in the hammered section! If you look at S2578B which is an Elizabeth I sixpence, singled out in Spink simply for its bust change to 6C. However, there are 12 major obverses and 38 major reverses for this Spink number, separated by PM's, overmarks/over dates/unmarked etc. I really don't think you can mix up tuns, hands, anchors and cyphers in the same Spink number and ever hope to arrive at a standard value?The Spink guide for the above number 'feels' like it could possible represent the total sale value of coins sold within this category divided by the number of coins included. I don't think there's any algorithm or decision to include only the commonest type? Using the above number again, I'd typically expect to pay a third less (or even more [less that is]) than the Spink value for a nice pm hand 6d, whereas, for an anchor or a cypher, you could easily double/treble the Spink book value! If I were to guesstimate the ratios of the various PM/overmarks etc. of S2578B that have been sold, and divided it by the number of coins themselves, I reckon you'd be somewhere close to the figure they quote! As I said, it's one of those silly eureka moments for me!