Hi John,
It does seem odd to find what would appear to be mechanic's equipment on an Electrical Repair trailer. I think the explanation is this, small generating and charging sets were being subjected to regular valve problems around the mid point of the War onwards, due to the introduction of leaded petrol. Airborne forces, working ahead of ground troops and their support, would have to be self sufficient initially, so to keep their charging sets operational, they would need valve refacers and seat cutters. Of course, not only for generators, but jeeps, etc as well, but it was documented by REME, the particular problems of small plant engines, having valve failure due to the leaded petrol.
regards, Richard