Ag or Cu-Ni? Isle of Man 1974 Winston Churchill crown

How do I spot the difference between the silver and the copper-nickel versions from pictures alone?
I know how to identify silver if I have it in my hand, but I don't.

I would have thought there must be some small difference in the engraving.

The Numista entries are no help since the same images have been used for both entries:
Cu-Ni: https://en.numista.com/catalogue/pieces22311.html
Ag: https://en.numista.com/catalogue/pieces22442.html

Referee informed

There is no difference in design for the silver vs the copper nickel. In early years sometimes you would find that the reeding on the edge was finer and smaller on the silver versions. Also, for some of Isle of Man's early crowns, the copper nickel version would be in unc condition, while a silver version was unc or proof. In this situation, the image is of a proof, so the image is for the silver version proof. I hope this helps you to attribute your coin.

Thanks Oklahoman.
I interpret your reply to mean that there is no way to distinguish the two from photos alone unless the coin is obviously a proof, in which case it is silver.