Following the excellent advice from Mark and Karl re converting A UK 50hz Zeddy To 60hz USA spec( Iwould say PAL and NTSC but Mark would shout at me!! )
I bit the bullet and made the conversion, programmed a 2764 EPROM with the tree Soft code lit the blue touch paper and stood back.
I should add at this point that the reasoning behind all this was to see if the original USA Forth ROM exhibited the same visual anomalies as the copied UK version.
Sad to say it does. The screen display is still too far up and to the right and when the screen is inverted it still has the far right column not inverted glitch as per the UK ROM (See picture).
The only difference seems to be the copyright message,both ROMS seem to function identically.
As both these ROMS work perfectly in the emulator Eighty One I can only conclude that the emulator or both ROMs have been tweaked somehow to hide these visual anomalies and in the absence of an ORIGINAL non copied factory ROM and,in the case of the emulator,Mike Wynne to tell us any different I will assume the ROMS graphical glitches are standard for this firmware.
Many many thanks to Karl and Mark for the info which encouraged a rank amateur like myself to have a go at a tiny bit of hardware modding.