The old fixed frame had issues with rear tracking tracking, it was steel and basically you set it with a long pipe or rod stuffed down the axle hole. If you look at the AL model posted in the video above it has tensioning rods meaning you can be precise and don't have to strip the axle out whenever you want to reset it.
Because the boom slopes up, back to front, changing the length affects the effective toe-in/toe-out as the rear wheels lean in. Getting that right makes the trike easy to ride and wrong makes it drag all of the time. I spent a productive two hours setting mine up when I got it, well worth the effort in reducing tyre wear and gaining speed.
The tensioning rods also act as the mount for the hub gear.
On my Rohloff there two chains, one is the pedal drive, and the other drives the rear wheel from the hub
The unit slides along these tension bars and is then held in place with a tensioner (I retrofitted a second) to tension the drive chain of the hub
The only real difference once set up is that you can alter the recline of the seat on the later models, but not the earlier ones
I did note on the yellow trike there is a release at the side of the seat so there may be some adjustment. The small black fitting at the rear of the frame is also the fixing for the Ortleib Hase bag.
I use standard lights mounted to the rear of the seat, but if you use the seat light fixing, reclining does affect the rear visibility by moving the light
