On the Southern, end filler pipes first appeared on Bulleid's coaches and were retro-fitted to Maunsells and Pullmans from the late forties. It COULD be that your external type fillers were, similarly, retro-fits of coaches which started out with top-fill only ? [ OVSB would have been involved with Gresley's coaches of course - but I'm not trying to imply any significance with the filler pipes.]