Well here is a photo of 2 breeding pair of Silver Appleyard. You can see why I think mine is a she-he!
Thanks for all your responses. Everybody seems certain the curl feathers define sex.
Because this duck is part of 2019 spring hatch and none are laying yet, I will wait for an
‘egg’ or ‘no-egg’ event to conclude the mystery.
Its possible he's just not old enough to have his true "nuptial colors" yet, that's what the green head feathers in the picture you're referring to is. But you can see yours does have the neck stripe. It took a while for my khaki drake's head to turn green.