The bushing doesn't appear to be a problem: If you look closely you'll see that there's a black rubber/PVC grommet where the cables enter the top panel.
It's not unusual to parallel two or more units like this, but I've just noticed that the two sets of tails both go directly into the top of the RCD. I'm wondering whether someone has trimmed off some of the strands on each tail in order to squeeze two cables into each terminal.
There are still plenty of 60 and 80A (and even some 40A) services still in use, but 100A is pretty much the norm these days. It's not standard practice to loop the mains building to building here; each house usually has its own spur from the mains under the street or to the pole. The service cables are probably 25 sq. mm.
