I think you are right - they have probably put in alternative firmware for Option H-14. *IDN? gives a firmware load of 3504A00166.USA_H14.1A. So it looks like a special version.
I've found the firmware binaries for another version online (
https://github.com/willhb/HP-58503A-Firmware).
And I've ordered some 29F010-120JI flash chips. So I'll give new firmware a try while not destroying the old!
Even though mine is marked Rev C and the binaries refer to Rev A.
My only issue is that my HP85803A has the "Option 1" front panel display as well. This may also need different firmware - we'll see.
HP did issue firmware updates but they have vanished, so I can't just do a serial port firmware update using SatStat (but thank you Didier -
http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=02_GPS_Timing/SatStat).
Does anyone have the original HP firmware upgrade file, rather than the flash binaries?
Or does anyone have binaries that are definitely for a version with the front panel display (Option 1)?
-Jeremy