Thanks for all the replies - it would appear that I do not have lamps in need of repair, but rather this WAS a design feature. I'm not really suprised some used the outer lamps for stop/turn while others (like mine ) used the inners (did they make any two of these trailers alike!?!?). Doug's and my trailer are very close in serial number so maybe the same guy did the wiring? Another member PM'ed me wondering if those outer feeds without pwr might be hooked to the non-used 7th wire on the main trailer connector - a potential dedicated stop circuit. I want to redo the wiring where the main connector cable enters the trailer and do away with all the original marettes, switching over to junction blocks. (I did this on my 1300, much easier to add extra circuts such as for chargers, stereo's etc.) I should be able to trace any unused wiring at that time and will post what I find.
As far as led's go, I had changed the lights on my 1300 to led's and loved the results - much
more visible and changed the whole look of the rear end (unfortunately no pics of them actually turned on...).
For the 5500 though, I have been looking at panel-style led bulb replacements- an adapter for the existing bulb socket with wires that feed a flat panel of 36 led's - specifically designed for applications where the bulb sits at a right angle to the housing like on our trailers. This would allow me to use the existing housing/lens and retain the original look of the trailer. Found a few on Ebay, but am going to check out the local trucking distributors to see if there might be something local. If not bulbs, they may have assemblies that I could be happy with - again, I'll post what happens.
In the short term, I was able to get a bit more light
out of the present lamps just by polishing the reflectors inside the lamps - 28 years of moisture and dust had tarnished them pretty bad. A good application of the same stuff I use on aluminum trim - Never-Dull - made a world of difference.