The mysterious HR16B triple layer board

[Circuitbenders]

Attached to this post there are two photos of the front and back of the main board from an HR16B i'm working on for a guy in the US.
Take a careful look..................... notice anything strange?



Thats right, there are no circuit traces on either side of the board! No links, no connections, nothing! Both the front and back of the board consist of a massive copper ground plane and nothing else. Every solder point is completely isolated from either side unless its a ground connection.
That naturally suggests that it must be a triple layer board with all the circuit traces in the middle layer, but why? If you're going to have all the circuit traces on one layer, why do it in the middle layer? In fact why have a middle layer at all, why not just put it all on the bottom layer.
I can only assume that it must be all in aid of shielding the circuit from RF interference, but surely that can't be cost effective. HR's from the US always have more in the way of interference shielding but this is ridiculous. I'm having some difficulty even finding the clock signal for the ASIC so i can use an LTC 1799 on this one!

Anyone else seen anything like this before, or has any kind of explanation?

[electoyd]

that's bizarre-i've opened several but never seen that before.  You do have to wonder what stage that was at in the hr16's career, what OS did it come with?

[Circuitbenders]

looks like v2.09 according to the sticker on the EPROM. I think that was the last one wasn't it?

In that its a HR16B i assume it must have been one of the last ones made. Maybe they were experimenting with new production processes or something.