Dog doesn’t look “huge” to me; I think 120lb is a bad estimate. I’m guessing they guessed and did not put them on a scale! The dog is too short and not fat enough to weigh that much.
“I adopted him from a human society over twelve years ago as a puppy, he was represented as being half Great Dane and half black Lab.”
Yet somehow you ascertained his exact ancestry although obtained at a shelter? I’m curious how that happened. How did you get exact measurements of AmStaff/Lab/Golden/Cocker?
Actually all this proves is that it can be hard (not always) to figure a dog’s background.
It’s a bit of a story. About four weeks after I adopted him, I took him to puppy basic training, where the instructor told me he was part Pit. I didn’t do anything about it then and didn’t return him because he’d won me over. Then, a couple of years ago, I was dropping him off at the vet’s and ran into a full blooded AST that looked almost like my dog, especially from in front. That got me going, so I had the vet run a DNA test on him, hence the exact breed structure.