Out of curiosity, I wnted to open one of the latest units of HP12c that I have received. This is a normal unit, made during HP's tenure in the Philippines (I think the actual producer is Kinpo, but I am not sure). Let's see what we find within!
When opening it, you have to remove the 4 feet in the corner. There are 6 screws to unscrew (do you find the sixth?). Trying to open it without unscrewing all will destroy the case.
The sixth screw is below the right battery. Careful!
Once opened, both sides are united by a three cables that carry the power from the batteries. The circuit, on the other hand, is visible in the back of the keyboard. It is very concentrated around the main processor. There are a couple of copper pads that I don't know what they are for. There are also a couple of small springs that are exceedingly easy to lose (This is common to other Voyager calculators of all vintages)
The white band below is an adhesive. again, I don't know what it is for. I have removed it and found nothing to protect below it.
The 6-point connector is a POGO 6-pin connector, but it is connected as a USB interface, as well as a reset device, by shorting specific pins. The problem is that this connector is completely non-standard. There was one produced long ago for the HP15c Limited Edition, but it has a completely different connection schematic. If you have one from that time you can modify it - but it won't work as is with the new calculators.
Is there not another alternative? Yes, it is. In the circuit there are provisions to solder a true USB nano connector. Where? in the right side of the circuit. Here:
I don't know the exact part number that can be connected here. You'd need also to break the case, or have opened each time you want to change the firmware. If you look into hpmuseum.org, you'll find that there is people around still interested on Voyager firmware mods. You could eventually want to repurpose the calculator for whatever type of calculator or functions. You have, though, the wrong keyboard for it. You would need to consider creating a silicone film with the lettering of whatever you want.