Interesting, but nothing new or novel. As "bn12" said, it's just a sideways Luger.
And how is a "sideways Luger" NOT novel? One certainly doesn't see them everywhere, or often.
Clearly the action is more then a bit "Luger" inspired, but there are features not direct Luger copies, or "just" a Luger turned 90 degrees.
The toggle action design is quite efficient, and when properly constructed, the "knee" is locked, strong enough to handle rifle pressures.
I believe the first gun to use the toggle was the Maxim gun. (yes, the first "real" machine gun) Borchardt took Maxim's toggle, turned it over and made a pistol out of it. Luger took Borchardt's pistol and made it better, and became one of the iconic firearms of all time.
Turning a Luger action on its side, scaling it to fit a .357 Mag round, creating the mag to feed that round, and getting it to work is no trivial task.
Making a "home made prototype" is not a trivial matter, either. And in 1970!
I can see why the seller recommends not firing it, its a one of a kind, handmade prototype, with no proven history of being safe, and of course, if you break something tis broke for good, and its value goes down.
Neat gun, good to see I'm not the only one who has considered using the Luger toggle concept for something other than 9mm.
I've never gotten beyond pipe dreams and a few sketches with my versions, good to see someone actually built a (working ?) prototype.