A mystery then !
I've ordered some push fit lamps (I'll soon have every 7V 0.1A lamp ever made...) to see if the holders are meant for them instead - there is no actual thread in the plastic.
The only other possibility is that the case work panel is not in the right place - the front looks fine, so I'd be surprised, but you never can tell.
Time to scratch head and have a pint..
and hope for divine inspiration..
Kevin.