I have never heard of an A-bolt-III, an A-bolt II yes, but a III , nope no such beast.
As for the push button you speak of that when depressed with the safety on allowing to open the bolt a remove round in the chamber is the X-bolt or the current generation of the Browning bolt action rifles.
A-bolts are still available , but in limited calibers and models (Medallions and/or Hunters)
$750.00 is steep for a used gun and yes I understand that you are the second owner and it has never been mounted or fired.
Leaves a few dollars to for wiggle room.
WSM will be around for a long time to come.
Tight Groups,
Rob