Checked, no "O".

There is the possibility the thing with your IIRs is the feet marking was an aftermarket addition - just as what appears to be a very professional job with the sync addition to mine.

Another theory would be the letters refer to slight variations within the model (e.g parts from different suppliers?) Or different "Supervisors" or Plant managers in charge of the various production runs? Somewhere there will be a record, it would be quite nice to find out some more information about the background of the company - Wonder if there's a German equivalent of Companies House that would have the records.

I'll dig the digicam out and get some larger images later tomorrow.