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Thank you very much both of you. Coenia palustris it is then.

It is slightly embarrasing to admit it but I was looking for the postpronotal in the opposite end of the dorsum (i.e. on the postalar callus) :|
The flies I collected have two short but rather strong bristles on the postpronotum. One has three.
At low magnification the colour is dark greenish with a hint of bronze. No males are sporting large genitalia.

I'll write Coenia palustris on the labels.