And it's fair to say that Bull's revision of ESC was not much of an upgrade. Missing items, items added that you would normally not include as a separate listing such as giving a prooflike specimen a different number, a provenance section at the back which doesn't match the ref. no. in the main body of the book. It basically wasn't proof read, or at least not after everything had been added. I list ESC references with the Bull number followed by ESC 5th edition numbers in brackets, as at least the only fiction in that edition was the rarity attribution.
Davies is far better than Bull for varieties, with the caveat that it only covers 1816 onwards, and as you would expect is missing some later discoveries.
Every reference volume has inclusions and exclusions relative to another book. You really need to buy the lot and sort out a proper list for yourself. It isn't that difficult because the vast majority of items are present in all and correctly catalogued.
My personal view is that a concordance book would be a wonderful addition, but the chances of getting collectors to buy it are probably south of zero. It would also be gargantuan, as essentially you are trending towards an entry for every die ever produced, cross referenced to every book that has been printed on the subject.