Okay now we're into match making.I think Stanley could indeed be a runner - pet quality, like my girls. He would be a nice match for my Zehn girl. She has broad shoulders, cleavage, short neck, and sassiness that just won't quit!
This is the third (that I can recall) case of a duck with swelling in the body this week. What is going on? Quackers was likely something like oviduct infection, and we don't know about Stanley and the other one right now.
My thoughts for possibilities are infection (fungal, bacterial or parasite), including possibly out of balance gut flora which would be something like a yeast infection.
What are his duck to duck relationships like? Is he picked on? Has he experienced any losses recently? Ducks grieve, so I would not rule that out.