Cam, I have an 'Export' version of the Series MM WSM and it states, ' The headlamps are of the "dip and switch" type, in which the nearside lamp beam is dipped and the off-side lamp extinguished on operation of the dipping switch. On some exported models the lamps are fitted with double-filament bulbs in order to comply with the regulations existing in the country concerned.' Hope this helps.
Bill Hewlett
Oxon & Berks Branch Chairman - MOT-UK Organiser (see http://www.blurb.com/books/1518384 and http://www.blurb.com/books/2422813)
Oxon & Berks Website: http://www.bucksinfo.net/mogbox/
Thanks Bill. It confirms what I now think and that is that ALL the cars are wired for double filament bulbs but the home market ones were simply fitted with a single filament bulb on the off-side. Makes sense.