The key to understanding the British manufacture of the finial comes not so much from the alloy, but from the ratio of the impurity elements of cobalt and nickel. On pps. 261f, Northover says (referring to the Thurrock potin coin type):
"The composition is of a bronze with an important cobalt impurity, with Co>>Ni and with iron, arsenic and silver as the other significant impurities. This impurity pattern is highly characteristic of metalworking in southern Britain in the La Tène Iron Age, and can almost certainly be associated with a copper source in south-west England, that is the metal itself is British in origin. ...Analysis of some cauldrons at La Tène itself showed the same bronze type in association with iron of potentially British origin. The high cobalt impurity bronze is uncommon in most periods so there must be a strong presumption that it was exported from Britain. ...The location and nature of the Thurrock hoard must also lead us to suspect that the potin coinage was a product at least of Essex as well as Kent. However, the bronze type is more characteristic of the south-west than the south-east although it circulated there as well."
Long after the above paper was published, Dean Crawford, an English metal detectorist and Dobunni tribe specialist discovered an as yet unpublished site in southern Worcestershire, which also yielded a number of Thurrock potins. The site was duly reported but has yet to be excavated by archaeologists. The site contained Dobunnic silver coins as well as the potins (and also later finds down to the last part of the 2nd century AD. suggesting metal recycling had taken place there at least at a later date than the original coin deposition.)
The coins appeared to have been cast deliberately in a scatter on the original surface of the ground (Crawford, pers. comm.). It was not a plow-scattered hoard. There was also evidence of feasting with a large number of animal bone remains. Dean sent me a representative collection of the metal finds, including one of the Thurrock potins which I had analysed at a U.S. commercial XRF lab. The Co/Ni impurity was recorded as well as the other significant impurities of Fe, As, and Ag all being within the same ranges as Northover gave for his four analyses of the Thurrock potins.
The nature of the Thurrock potins is currently being revisited by Mark Fox of Michigan, and the first part of his research is published in this month's issue of The Numismatist (American Numismatic Association), p. 37-43.
There can be no doubt, whatsoever, that the metal content of the finial belongs in this British alloy type. The only important difference being that the finial had lower levels of tin -- but like other artifacts in this alloy group, it is bronze and not potin. In fact -- even taking the averages of the Co content, it is slightly higher than is cited for the metal type by 0.003% in this element.
Finally, this analysis gives material confirmation of Martyn Jope's statement on page 1 of his Early Celtic Art in the British Isles about the primary development of British early Celtic art:
"The initiating stimuli for this rise evidently came from Europe, yet at the crucial time, the fourth to third centuries B.C., we can point to practically no imported pieces that might have served as potential exemplars; the new ideas and skills must have come largely in the minds and hands of men with a considerable experience in distant ateliers."
It is to the memory of Martyn Jope, that this installment is dedicated.
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