Stig,
Yes, that is why I speculated on 403. But you may very well be correct an it is neither. It just seemed an odd thing for the individual writing on rthe postcard at the time to write, leading me to speculate, highly possibly incorrectly that 403 was meant. ut I agree really unknowable anfd I just posted it for the sake of completeness. As a precaution I shall scourge myself for possibly having posted a civilian aircraft
Regards,
Clint