45 MM, 17.83 G Justinian Follis A / N / N / O And XII, 538-539 Ad Nicomedia

That's as big as they get. There has been forgeries of this coin from ANNO XII NIK, this one looks OK.

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.

So a man I don't think I have dealt with before has just written in below. (Forgive me sir if I have.) I have asked him to be so kind as to post on this forum. What he just wrote is: (I told him I can pull this long before the 7 days not to worry. But I'd like to see what he has to say first to at least learn from this experience.)

--->"Hi, this is a well known cast fake from the Middle East. It's underweight for the type, which is almost always 20g+ and you can also see traces of filing on the edge. You should not sell it as original.
"<---

Yes, this a well-known fake. I actually bought one myself at a flea market 30 years ago. The two below I found with a quick Google search.

(Edit) There are also several examples in the fake reports at Forvm. Applied patinas vary. All seem to share a similar flan shape, a mushy cast appearance and a little spur at 3 o'clock on the obverse margin.