I have never seen factory-original speakers mounted as you describe. It sounds to me as if someone took their time and did a very tasteful speaker installation. I, too, love having good-sounding music in my cars, and it's always a struggle to achieve that without damaging the coach. In my '76 Victoria I mounted 6x9 speakers, in enclosures, high up in the partition, but didn't cut big holes IN the partition. I drilled a total of 8 holes through the wood (and fabric) above the glass, and bolted metal angles to the partition. Then I lay the speakers across the angles, facing downward, and used small screws to hold the speakers into place. It pained me to drill through the partition, but unless you're looking upward toward the top of the partition, you never even notice they're there.
I took this car to CLC Kansas City last year, and utilizing these as rear speakers, and the original two Cadillac speakers as fronts, cruised with a great sounding system. The source for the CD & radio is actually a Pioneer unit with remote control that sits on the hump on the floor. Some years ago I stumbled upon a set of brackets, no doubt intended only for a CB radio, that would allow the CB to sit on the hump on the floor. I mounted the CD/radio unit on these brackets, and fabricated another set of metal angles that allowed me to mount my CB radio above, just in case I needed it. A couple quick plug connectors to the rear allows me to move the whole thing from car to car. All the speakers just stay with the car. When I get time I'll photograph how this looks. Tom