The outboard banjos appear to be a ring gear driving two pinions to the front and rear wheel on each side. Big gear driving little gear puts a hell of a lot of torque on the big gear axle, - judging from the size of the ring and pinion (~4:1), about four times the torque. But that appears to be a pretty stout shaft. Reduction in the wheel hubs would mitigate that torque requirement, but from the size of those hubs, maybe only 2:1.
So, if they wanted a 5:1 ratio from engine to wheel for reasonable road speed, the center ring and pinion would have to be 10:1. No way that fits in the carrier depicted, so the transmission probably has something line 2:1 reduction in high gear.
All pure speculation on my part, but I am fascinated by such contraptions.