It looks like someone filed the numbers off the long chip next to the crystal and the ADM232 chip. That might be the A/D converter chip. It could also be a uprocessor. I'm afraid there is really not much there that we didn't already suspect, ie: instrumentation amplifier , op amps, RS232 converter. The presence of the crystal and a chip with the numbers filed off is telling. Someone didn't want anyone reverse engineering this box.