DVLA vehicle check reveals it to be 387cc.
Most likely an NSR400 engine with either a dummy 4th pipe or maybe the top one bifurcating into two end cans under the seat.
I initially wondered if it was a 4 cylinder RG400 fitted,for some reason,but that wouldn't be 387cc as the capacity is 397cc on them.
Must be daytime only MOT given the lack of lights but I don't fancy having no mirrors or indicators on today's roads