Tail lights, brake lights, turn signals on '75 22'

[mendosoft]

I've had some work done on our trailer and during the work we noticed that the 4 rear lights were wired as follow, from left to right:

Left outside - tail light, brake light, turn signal
Left inside - NOTHING?!
Right outside - tail light, brake light, turn signla
Right inside - NOTHING

We've checked the wiring at the tow vehicle an all looks fine there. So we're wondering - how are these SUPPOSED to work? It doesn't make sense to hae the two inner lights do NOTHING? There must be a catch...

Thanks in advance for any insight.

Steve