I like the photo sequence.
As a person with just one dog, who doesn't visit dog parks, I don't know much about dog play.
I thought they were trying to kill eachother, until I got to that last photo!
I've noticed German Shepherds and Huskies tend to play like this. Huskies especially love to growl ferociously when they wrestle.
Jack loves to growl like a monster and then wrestle, but he'll put himself in the deepest play bow possible at the start so the other dog is absolutely reassured that he's not actually going to try to kill them. Even though he sounds like it! It scares other dog owners, I think.
The key I've noticed is looseness in the body language. If dogs are moving about a little loosey-goosey, bouncing around and behaving in a very exaggerated manner, then it's playing. By exaggerated, I mean ridiculously loud growling, big wide-mouth displays with loose lips (and pushing at each other's heads in the process), sloppy movements, big jumps and almost slow-motion body checks and slams. That's playing. They're purposefully over-communicating and telegraphing their next move and giving each other LOTS of time to form an appropriate response. What looks fast to us is actually very slow to the dogs.
Another key is if they stop playing, and they look away from each other then play bow again, or simply resume play immediately. Like taking a breather between rounds of wrestling. And after the play session, you'll know your dog was inhibiting their bite and following the "rules of engagement" appropriately if the only mark on the other dog is lots and lots of slobber.
However, if everything looks very, very tight and stiff and still, then the ensuing fight will be serious. The mouths will be closed more. The movements will be even faster. Their ears will be back or sideways. Their tails will be lower or straight up. They won't aim for just the neck and head. They'll go for other body parts.