I had a second-hand RS-133. Here's the only picture I could find. I replaced the battery pack myself. Not very good at this kind of things. Please don't judge the workmanship. It worked fine in general. I remember the measurement function was lightning fast and steady. Very impressive. One thing I don't like is the manual didn't mention the calibration procedure, so you might need to send it back to the factory to get it calibrated.
There's another PCB under the one you can see. There's a switch-mode DC-DC converter at the upper right corner, which I suppose powers the system, and there's another one at the bottom. You can see the inductor there. I think this is probably used to generate the output voltage. Not sure if it's a good idea to put a smps so close to the output ports. I couldn't find the voltage reference chip. It might be under the copper shield, but I didn't open it. Another thing I could remember is that the calibrator couldn't charge while turned off, which is very strange.