The edge looks way too good to be true, to actually be true.
I am unaware of any 40 nummi being as large as 43mm diameter.
Although there is only a slight possibility, I would be tempted to investigate if it is not a coin, but a contemporary medallion. The other very slight possibility is that it may be a sort of contemporary 'proof', used as a sort of quality control. All of the detail is evenly struck up, on a flan of constant thickness.
However, both investigations would be started from an initial position of great suspicion.
After checking with fake ancient coin data bases,
perhaps an inquiry with the British Museum or the Bibliotheque Nationale would be a good place to continue. If nothing else, there would be some very experienced experts there, that may be able to help.