I agree with Janner, I have upgraded a number of so called PLC's like this, (think I did one that looked almost identical) never managed to get the software (probably dos anyway), I only had one where there was a ladder listing in the panel so quite easy to migrate with a few extras.
Your only hope if you cannot get a listing or software is to do it the hard way, plenty of time talking to the operators or if possible watch it working and if no drawings tracing the I/O. That is assuming the customer has the money.