I've had Labradors since 1967 and they all did that, also a border collie I had did the same and would "smile" upon command. I also did behavioral research with Beagles and had a colony of about 250 dogs. They, too, vocalized in that pattern. And finally, I did research with a pack of 80 wolves, in a field station setting, and they, too, made the same sound.
You certainly are the expert. I have owned probably 50+ dogs in my life including several Labs, Lab Collie mix, Yorkies, Jack Russels, Shelties, Rhodesian Ridgeback, Scotties, birddogs, Bassets, Bloodhounds and others including mixed breeds of all kinds and unless I just don't recognize the sound I have never had a dog that made it. Cats, on the other hand do it all the time.