Duck ID

Off-coloured Mallard

Green head and white bib = Cayuga-type.

I think the idea of a 'Cayuga type' with a white bib comes from Peter Scott's 'Coloured Key to the Wildfowl of the World', which mentions it.
A Cayuga is a big solid black [Mallard-descended] domestic duck. Think Aylesbury & colour it glossy greeny-black all over. It does not have a white bib.
If this bird was a Call Duck it would be described as Black Bibbed. However, its general shape is of a wild Mallard [and this colour pattern is common among free flying populations], so it's a Coloured Mallard. Sorry to go on.

Yes, I learnt that definition so long ago from that Key that it stuck. But in wild/semi wild Mallard populations it does seem the black/green birds usually have white bibs too. I guess he overlooked the fact the true domestic breed Cayugas, as you describe, don't actually have these, being dark all over, and that it was the nearest description he could think of.