Twin contact upper bulb sockets were the result of an Australian Design Rule (1972) that required either Amber or White Reversing (Backup) Lights.  Thus the 1157 bulbs in those sockets.  They had to illuminate when reversing with motor running or just with ignition on and reverse gear engaged but could not over ride turn signals.

Are the second set of filaments connected to anything? I like to modify my taillights to use dual filament bulbs, and I connect the second filament in both bulbs as brake lights. 

A local auto parts supplier Princess Auto (Coquitlam, B.C. store is closest) advertised LED trailer lights in its recent flier.  ID'd as 1157 replacements my transcontinental co-driver bought a set for me and I put them in the OZ Mini Van today.  As they are on/off they work just fine as parking and brake light "bulbs".

Interesting that the upper bulbs (Turn Signals) in the rear assemblies are also twin filament 1157's and the socket is designed for only an offset pin, twin contact bulb??

Now I need new lens to base gaskets too.