Rhododendrons have black anthers. The edges only look serrated on the whole because of the ruffles in the also on the whole rounded edged petals.

Do Dianthus have black anthers? One picture in 99 can be found that do. Finding the one in a hundred in 1 picture will not solve this quest!

The photos show a range of colour of anthers, with mature ones being purple. Yes, many Dianthus species have anthers that are dusky purple when mature, as on the plant in question.
These photos show clear serrations at the petal edges, which was the one thing that was not totally resolved on the previous photo towards a double-flowered Dianthus barbatus ID... but now it is.

So it's a Dianthus barbatus (double-flowered)?

I'd say so.

Thanks.

Thanks for the extra photos, jmorth!

I'm definitely sticking with my original I.D. of Dianthus barbatus, no question.

I promise never to speak again on the subject now,.... and I can't believe you are all so wrong!

Dianthus barbatus or Sweet William flowers look absolutely nothing like this plant clearly!!! The flowers are far flatter for one. Holy Moly!

Not when it's doubled and that crowded, no room to be flat.
and it looks like this one, equally as crowded:


and the leaves beneath it in Paul's last two photos are clearly dianthus.